As I haven't donated for myself before i'm not 100% sure but I am fairly certain that you can just type in the prepaid card on your account and then pay for the donations like normal.
I know that you can pay with a regular credit card as I have done that before. I would suggest doing that, but, I'm not sure with a prepaid card.
Like everyone before has said. When you go to check out look underneath the paypal login for that page and you will see the ability to use a card. You can use prepaid cards too as I have done this before!
You'll still use paypal to checkout, just don't use the option to connect to a paypal account. Also; if it's a prepaid card it must usually activated on a site that card will tell you before you can use it.
If it's a visa, mastercard, maestro, etc. Then you can pay using paypal. Paypal lets you pay with creditcards. (I myself use a prepaid visa card for donating) Make sure the card is activated on the website of the card. Also moved to help & support.
Stin, it isn't very difficult to make a paypal account. I am a minor, and use visa gift cards to go through the process. If you have an account (only takes about 2 minutes to make) it will make the process much easier. I have donated over $1,500 this way.