Again let me state the steps so you can try and figure out your issue.
1. Make sure you have your usergroups all imported and your not forgetting any by refreshing usergroups in Admin. Set what groups can pay and which can highlight italic etc.
2. In Admin => Edit Categories You set which usergroups have permission to access upload etc and what the price is for placing an ad bold italic highlight.
3. In Admin => Payment Options set your paypal email and set paypal as the processor.
As long as a user who is assigned as payment =yes uploads to a category that has a charge for an ad etc they will be prompted for payment.