"Add Class" greyed out on a dialog in the resource editor

I have some dialogs I have been working on in the resource editor (Dev Studio 2008). In the past I could right click on the dialog and a menu would open and it would have an option, "Add Class" which would create the class backed by that dialog resource.

Now it is greyed out. I have tried making new dialogs, the generic template ones, and in those it is also greyed out. Can someone tell me why and what is preventing me from creating classes for these dialogs?

This is in a Dialog based application if that matters.