I just noticed in MonoDevelop an odd quirk with code completion and Stetic. If you add a widget then go and try to refer to it in code, it doesn’t show up in code completion until you build the project. So, simply add the widget, build the project, then go into the code and you’ll have code completion for the new widget available.
Bizarre, but whatever. It’s just one of those little quirks that you need to get used to.