Go here: www.paypal.com/myaccount/wallet/add/card
You may be prompted to Log In, do so.

If it doesn't take you to that page, click on it again from here.
The page should look like this.


Also, checking your Resolution Center in PayPal might not be a bad idea, because it may tell you you can't use your Debit Card for some reason etc.
It can be found here: www.paypal.com/cgi-bin/webscr?cmd=_complaint-view&nav=0.5