Right now the method for debugging Media Center applications is pretty much to use Visual Studio both for jscript debugging in HTML apps and managed code for add-ins.  I know some people have had some problems getting this working and I'll try and post something on that later.  There's been some talk about how we could make debugging Media Center applications in the future easier, if you ideas for things that would make it easier for you, i.e. tools that would help, logging or anything else, leave me a comment and I let the rest of the team know.  I can't promise that we'll use any of the suggestions but we'll certainly consider them and it well help us decide what we need to do.