I can only tell you how to do it if you have a U.S. PayPal account, as it seems that certain stuff on the PayPal website is switched around depending on what country you are in.
But here is what to do ... first, go to PayPal and login. Then click the dark blue Merchant Tools tab near the top of the page. There should be heading a lil farther down on the page that says Website Payments in bold, with mini credit card icons below it. The last line of that section should be a link to Donations. click on Donations, and then fill out the form. You'll then be presented with some custom HTML code to put on your site.