Unfortunately, I don't have a Windows 7 phone to test against, so I can't be sure. It would help me get to the bottom of it if you could tell me any sort of error message that it gave when you tried to add it.
There's not really anything too fancy about the RealmFactoryCore.dll that stands out in my mind as a problem for the phone.
Hmm… unless the phone is only using the client profile, or is only using the .NET framework version 3.5 or something. If that's the case, there may be an easy fix.
But yeah, the place to start is if you could tell me what any error messages say that you see when you try to add a reference. Let me know….