Ah… by default the class validation won’t let you derive from things that aren’t available to the default game module.
We probably need to move the module selection combo to the first page, so that you can specify which module you’re adding the class to.
The class validation is already linked to the selected module though, so you might be able to select “None”, go to the next page, then select your module from the combo, then go back and actually select the class you want.