Domein place on WordPress

Hello,

I bought a domein name, and want to transfer it to my free wordpress.com site. Can somebody help me with this?
I tried domeinmapping in wordpress, but when i try to pay this, they ask for my creditcard. But i want to pay with paypal.
I don’t understand how this can happen, because I click pay bij paypal and i have money on my paypal to pay.

Thanks! Yvonne