Run a Groovy script step to register your JAR file each time you open SoapUI before you try to connect to any database. Once you've run the Groovy step, you won't have to do it again for that SoapUI session. Then test your connection.
Here's the Groovy for the script step.
import groovy.sql.Sql
com.eviware.soapui.support.GroovyUtils.registerJdbcDriver( "com.microsoft.sqlserver.jdbc.SQLServerDriver" )
This will at least get you beyond the dread "failed to init connection" error. I use jTDS so I can't help you with the connection string. You might want to consider jTDS because you don't have to embed a password in your connection string; performance is also (supposedly) better.
