I tried to open a TFS 2010 build template within the Visual Studio shell (the bit that gets installed when you put Team Explorer onto a PC) and saw the error “The document contains errors that must be fixed before the designer can be loaded”.
At the bottom of the screen it showed that all the underling assemblies could not be found.
The solution is simple, install a ‘real version’ of Visual Studio, I put on Premium. It seems that the shell does not provide all the assemblies that are needed. Once I did this I could edit the XAML with no problems