Oh I see! Ok, forget everything I've said before. I understand it now: you want to narrow your search by selecting one or several specific categories, right?
Unfortunately, that is not possible with build-in methods. Because that is a new feature and each catalog page, has already been assigned a specific function, that is the price that you paid for simplicity ;-)
But of course, since SimpleCatalog is based on ItemManager you are able to create an extra GS page for that purpose, and you will have to use API to write required function. If you have any questions regarding API, just let me know and I will try to help.
Unfortunately, that is not possible with build-in methods. Because that is a new feature and each catalog page, has already been assigned a specific function, that is the price that you paid for simplicity ;-)
But of course, since SimpleCatalog is based on ItemManager you are able to create an extra GS page for that purpose, and you will have to use API to write required function. If you have any questions regarding API, just let me know and I will try to help.