I've been using TFS on Mac for a bit and it has the same problem that I have using Xamarin Studio with TFS on windows. The files are in a read-only state (which with VS this works fine). I manually change them to read/write.
There is probably a better way but they(IT) won't let me setup an SVN bridge to use TFS from within Xamarin Studio. ....
Also side note the Eclipse plugin for TFS works great and gives you all the functionality you have in VS; I recommend switching from just the command line tool.