I have multiple projects in our solution that is in our TFS 2010 repository. I\'ve added about 5 new projects to this solution in recent days.
I have found that for one
Your project doesn't have appropriate Source Control Bindings! With the solution open, you'll need to go to File -> Source Control -> Change Source Control
to view the bindings for your projects. The failing one probably will show an "Unknown" or not binded status. You can then use that dialog to correct the binding and check in the project file.
Appended Alternate Solution -
Navigate to the Team Explorer -> Source Control
and manually add the files using the "Add Items to Folder" menu item.