It took me forever to figure this out.
I believe you need to have one of the Activity files selected and visible (like the .b4a or I believe one of the activity .bas) within the B4A IDE to serve as the target for generated members. The members that are generated will be placed into the file selected within the IDE. I believe if the file selected in the IDE is a class module or code module, then the Generate Members button will the grayed out.
Hope this helps.