Some details about the Psybuck System can be found here
. However, your specific question is not answered very well there. Basically, you click on the "Donate" link under someone ELSE's name in any of their posts (the donate button does not appear under yours to you, because you cannot donate to yourself. It is visible to everyone else, though, trust me.) Then, you type in the amount of Psybucks you want to give them, add a message if you want, click "donate", and the number of psybucks you typed in are subtracted from your total, and added to theirs.