Any post that contains variables or sections that are not recognized by UPG will not open in it. In cases when this happens, simply use ECEdit to make the changes you want and use UPG to compile the post and create the .SRC and .LNG files. Make sure your preferences are pointing to the correct Master.ATR file, etc. There are videos on YouTube that explain a lot of this stuff fairly well. It is worth checking them out to get a better idea of how everything works.
Sorry you didn't get a response when you first asked your question. There has never been much participation in this forum. If you have more questions along the way, feel free to ask here and I'll try to help out now that I'm getting back into CAMWorks.