I found a way:

auto view = new RadioMenuItem (group, name);
view.addOnActivate(delegate void(MenuItem foo) {this.selectNotepad();});

This is a bit awkward because view is a RadioMenuItem but since addOnActivate is defined in MenuItem, that's what the delegate parameter must be. Why doesn't inheritance make them equivalent?

I also saw an example that does:

view.addOnActivate(delegate void(_) {this.selectNotepad();});

It works, but I don't understand why.