Farseer 3.1 "officially" only supports XNA 4 and XNA 4 just supports Visual Studio 2010.
So either switch to XNA 4 / VS 2010 (Express)
Take XNA 3.1 / VS 2008 / Farseer 3.0
As Farseer is not that dependent on XNA 4 and does not use any of the new C# features (yet). So you might also be able to create a new VS 2008 project file and import all the Farseer 3.1 files. Content management changed quite a bit in XNA 4 so getting the
debugView and/or the Samples to work, would require some effort from your side, but it should be possible. If it is worth the hassle is totally up to you I guess ;)