I am playing a Chess960 as Black where i got to the position where my King is at c8 and my Rook at a8. I wanted to castle Queenside. Both K and R have not been moved, the squares between them as well as the R's destination square d8 are empty and none are under attack. but when i tried to put the R at d8, the program whishked it back to a8. I tried this several times w/ the same result. What am i doing wrong? Or am i misunderstanding the castling rules?

Try moving the king onto the rook. Sometimes that helps.

but the king is already on c8 -- it doesn't need to move, does it?

No, but moving the king onto the rook will tell the computer that you want to castle. Then the computer will figure out where the king and the rook should end up, which will result in your king not moving.

Thanks, Slack and Ichabod! I tried it in another game, and it worked.