Make sure you add HedgehogDevelopment.CodeGeneration.Extensions.dll
to either the GAC or the Visual Studio Assemblies folder:
https://github.com/HedgehogDevelopment/tds-codegen/wiki/Using-Extension-Methods
Or use the full path to the assembly in your directive.
UPDATE:
As an update to this, the extension methods have now been moved into T4 files so you no longer need to deploy the DLL to the GAC or Visual Studio folder. You can find the updated files in the Github repo.