Sorry, I've never been a member till today... how do I get the donate button on the right when I post a font?

Click on "My Account" at the top, then click "Donate link".
then you just have to enter your paypal email adress

FullerFonts, you don't 'post' fonts on dafont. You submit them, and the webmaster decides if they will be made available on the site. Typically, new batches of fonts are posted every five weeks or so, and those designers who have one or more of their fonts posted in a new batch will receive an e-mail confirming their inclusion, usually on the same day that the new fonts are posted.

~bito