Hi James,
You must be working off the old .tt files from the Hedgehog TDS Code Gen examples.
In those examples, there was a dependency on a special DLL (the HedgehogDevelopment.CodeGeneration.Extensions.dll), that would get built out to your Visual Studio 'Public Assemblies' directory. (See the README.txt file that was in the original Extensions project.)
There are upgraded .tt files on GitHub, which remove the dependency on this DLL, so you won't need it anymore.
You can either:-
2. Find the old DLL and copy it over to you Public Assemblies directory.
Note: If you copy over the old DLL, you will need to put it in the appropriate Public Assemblies folder. By default the original code gen example project copied it to C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\PublicAssemblies, but for Visual Studio 2012, change the 10.0 to 11.0, and for Visual Studio 2013, change the 10.0 to 12.0.
Hope this helps.
- Sean