try to take it from :
C:\Program Files (x86)\Microsoft SDKs\RIA Services\v1.0\Libraries\Server\
If you don’t have it goto - http://www.silverlight.net/getstarted/riaservices/
Pay attention that the Microsoft.Practices.Unity that by default installed in your Program Files folder is different from Silverlight Prism template .
Choose the one from Silverlight Prism template . I’m managing all third party or in a way special dll’s in an External.Libs for each project – this makes the project portable .
It has to be in our case 188.8.131.52 version.