After an extended absence, I returned to hacking customising TFS Process Templates today. Last time I dabbled in this space, the Process Template Editor was a standalone application. It's now part of the excellent Team Foundation Power Tool. One problem... I installed the Power Tool and couldn't find the Editor anywhere. After a quick look around, I learned I needed to install Domain-Specific Language Tools for Visual Studio 2005 Redistributable Components. No problemo... but still no Editor. I then repaired the Power Tool... but still no Editor. It was only after an uninstall/reinstall cycle of the Power Tool did the Editor appear on the Team menu of Visual Studio.
The moral of the story... RTFM and install the DSL Tools before the Power Tool.