In the "ATP Check Control", Select you ATP Group and on the left side click on the scope of check folder, this will open a screen where you can maintain all the relevant ATP categories to be used in the ATP scope of Check.
Now, how do create an ATP Category itself as a receipt/requirement/stock?
SPRO->APO->GATP->General Settings->Maintain Category
Whenever a category is created, you get to maintain if it a requirement / receipt / stock / forecast.
Regards
AK