Thanks. That's an easy one but that wasn't the problem. I had my project set as the default startup project. I'm not sure why VS 2010 was looking at Farseer for an entry point.
It may be something simple I'm missing in the properties or assembly file info of Farseer that doesn;t come over when you copy over classes manually as opposed to importing the .csproj.
I decided to just reference the Farseer .dll in my Windows_Phone XNA 4.0 project.
This is the error I received.:
Warning 3 The primary reference "FarseerPhysics" could not be resolved because it has an indirect dependency on the framework assembly "mscorlib, Version=188.8.131.52, Culture=neutral, PublicKeyToken=b77a5c561934e089" which could
not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0,Profile=Client". To resolve this problem, either remove the reference "FarseerPhysics" or retarget your application to a framework version which contains "mscorlib,
Version=184.108.40.206, Culture=neutral, PublicKeyToken=b77a5c561934e089". LavaRunnerPhone
It appears the mscorlib version that is used by Farseer in XNA 3.1 needs to be updated, then Farseer can be compiled and then you may be able to use the Farseer .dll in a Windows Phone XNA project.
I'm still trying but this may be deeper than my brain can handle.