If you have a transform with an extra '.' in the file name the Preview Transform command will not show up. For example if you have a build config named "QA.01" and then use Add Config Transforms to add web.QA.01.config then the Preview command will not show up for web.QA.01.config.
Should be fixed with commit 4c1994e. A new build is available with the fix at https://dl.dropbox.com/u/40134810/SlowCheetah/SlowCheetah-4c1994e819.zip.
I've just tested, works ok. Thanks.
The fix to this has been released at http://visualstudiogallery.msdn.microsoft.com/69023d00-a4f9-4a34-a6cd-7e854ba318b5/.