Hi Mor,
Your silverlight application always should be properly prepared for testing in TestComplete. There are http://support.smartbear.com/viewarticle/29603/ to do this including the one when you don't have access to the .xap file.
However, this procedure doesn't affect the control used in your app. If the control isn't located on http://support.smartbear.com/viewarticle/31621/, the only way to work with it in TestComplete is to use the control's native methods and properties. See the following topics for details:
- http://support.smartbear.com/viewarticle/29321/
- http://support.smartbear.com/viewarticle/27417/