well it looks like a lot of people make this mistake...
from looking at this page:
http://sourceforge.net/supporters.php
it seems like about 1/3 of the comments there make you think people were donating to a specific project (going quite a ways back), not source forge, but they got misled like so many people here. It is unfortunate. You could try to talk to paypal and report exactly what happened, sourceforge misled you into donating to them instead of your intended target. Especially the people who put eqemu in their comments, I think it would be bullshit of sourceforge not to refund it... But chances are, all they need to do is not reply to your email and theres prolly not much that can be done.