I have the same problem here up to version 5.0 (Nigthly). The Certificate and Password can be load and used for requests via "Preferences/SSL Settings". But if I take the same settings in the WS-Security settings, to use it with different environments, the error described from parwezwahid appears. 
Some more Message from the console: 
15:47:32,071 ERROR [SoapUI] An error occurred [failed to extract any certificates or private keys - maybe bad password?], see error log for details
15:47:37,078 WARN  [KeyMaterialWssCrypto] Using fallback method to load keystore/truststore due to:
Invalid keystore format
Please report bug!
PKCS12 detection failed to realize this was PKCS12!
------ ASN.1 PKCS Structure ------
oid1:    HIDE
salt:   HIDE
ic:      HIDE
bigPayload-length:   HIDE
All oids:
HIDE
and after press enter in the Password field of "WS-Security/Keystores" following message appear.
16:01:46,323 ERROR [SoapUI] An error occurred [failed to extract any certificates or private keys -maybe bad password?], see error log for details
java.lang.NullPointerException
        at java.util.PropertyResourceBundle.handleGetObject(PropertyResourceBundle.java:155)
        at java.util.ResourceBundle.getObject(ResourceBundle.java:387)
        at java.util.ResourceBundle.getString(ResourceBundle.java:353)
        at org.apache.ws.security.components.crypto.CredentialException.getMessage(CredentialException.java:111)
        at org.apache.ws.security.components.crypto.CredentialException.<init>(CredentialException.java:87)
        at org.apache.ws.security.components.crypto.CredentialException.<init>(CredentialException.java:62)
        at com.eviware.soapui.impl.wsdl.support.wss.crypto.KeyMaterialWssCrypto$KeyMaterialCrypto.load(KeyMaterialWssCrypto.java:323)
        at org.apache.ws.security.components.crypto.Merlin.loadProperties(Merlin.java:190)
        at org.apache.ws.security.components.crypto.Merlin.<init>(Merlin.java:140)
        at org.apache.ws.security.components.crypto.Merlin.<init>(Merlin.java:135)
        at com.eviware.soapui.impl.wsdl.support.wss.crypto.KeyMaterialWssCrypto$KeyMaterialCrypto.<init>(KeyMaterialWssCrypto.java:308)
        at com.eviware.soapui.impl.wsdl.support.wss.crypto.KeyMaterialWssCrypto$KeyMaterialCrypto.<init>(KeyMaterialWssCrypto.java:304)
        at com.eviware.soapui.impl.wsdl.support.wss.crypto.KeyMaterialWssCrypto.getCrypto(KeyMaterialWssCrypto.java:106)
        at com.eviware.soapui.impl.wsdl.support.wss.crypto.KeyMaterialWssCrypto.getCrypto(KeyMaterialWssCrypto.java:47)
        at com.eviware.soapui.impl.wsdl.support.wss.entries.WssEntryBase$KeyAliasComboBoxModel.update(WssEntryBase.java:277)
        at com.eviware.soapui.impl.wsdl.support.wss.entries.EncryptionEntry$InternalWssContainerListener.cryptoUpdated(EncryptionEntry.java:355)
        at com.eviware.soapui.impl.wsdl.support.wss.DefaultWssContainer.fireCryptoUpdated(DefaultWssContainer.java:382)
        at com.eviware.soapui.impl.wsdl.support.wss.crypto.KeyMaterialWssCrypto.setPassword(KeyMaterialWssCrypto.java:291)
        at com.eviware.soapui.impl.wsdl.panels.project.WSSTabPanel$CryptoTableModel.setValueAt(WSSTabPanel.java:746)
        at javax.swing.JTable.setValueAt(JTable.java:2740)
        at javax.swing.JTable.editingStopped(JTable.java:4722)
        at javax.swing.AbstractCellEditor.fireEditingStopped(AbstractCellEditor.java:141)
        at javax.swing.DefaultCellEditor$EditorDelegate.stopCellEditing(DefaultCellEditor.java:368)
        at javax.swing.DefaultCellEditor.stopCellEditing(DefaultCellEditor.java:233)
        at javax.swing.DefaultCellEditor$EditorDelegate.actionPerformed(DefaultCellEditor.java:385)
        at javax.swing.JTextField.fireActionPerformed(JTextField.java:508)
        at javax.swing.JTextField.postActionEvent(JTextField.java:721)
        at javax.swing.JTextField$NotifyAction.actionPerformed(JTextField.java:836)
        at javax.swing.SwingUtilities.notifyAction(SwingUtilities.java:1664)
        at javax.swing.JComponent.processKeyBinding(JComponent.java:2879)
        at javax.swing.JComponent.processKeyBindings(JComponent.java:2926)
        at javax.swing.JComponent.processKeyEvent(JComponent.java:2842)
        at java.awt.Component.processEvent(Component.java:6282)
        at java.awt.Container.processEvent(Container.java:2229)
        at java.awt.Component.dispatchEventImpl(Component.java:4861)
        at java.awt.Container.dispatchEventImpl(Container.java:2287)
        at java.awt.Component.dispatchEvent(Component.java:4687)
        at java.awt.KeyboardFocusManager.redispatchEvent(KeyboardFocusManager.java:1895)
        at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(DefaultKeyboardFocusManager.java:762)
        at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(DefaultKeyboardFocusManager.java:1027)
        at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(DefaultKeyboardFocusManager.java:899)
        at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:727)
        at java.awt.Component.dispatchEventImpl(Component.java:4731)
        at java.awt.Container.dispatchEventImpl(Container.java:2287)
        at java.awt.Window.dispatchEventImpl(Window.java:2719)
        at java.awt.Component.dispatchEvent(Component.java:4687)
        at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:729)
        at java.awt.EventQueue.access$200(EventQueue.java:103)
        at java.awt.EventQueue$3.run(EventQueue.java:688)
        at java.awt.EventQueue$3.run(EventQueue.java:686)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
        at java.awt.EventQueue$4.run(EventQueue.java:702)
        at java.awt.EventQueue$4.run(EventQueue.java:700)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:699)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
Could you please take care of it? 
Thx a lot!