I just downloaded Farseer and ran into the same problem.
Here's what fixed it for me:
In the solution "Properties" box in the bottom right of Visual Studio, the first property is "(Name)" and the value is "Samples XNA XBox360"
The second property is "Active Config" and I had to change this from "Debug: Any CPU" to "Debug: XBox 360"
Since this thread is from June, I doubt this is of help to moorster; but hopefully is of use to any other new users who try to search the forum