Satoshidice has a warning on their page that you should use a test amount first before betting anything big:

"Only use wallets that allow you to receive Bitcoin from the same address you sent from. If you're not sure, test by sending a bet for the minimum amount. If you get nothing back, then your wallet is not compatible. Note that some bets may require one confirmation before the win/loss value is sent back."

You neglected to follow the warning and therefore it is not MtGOX's fault that you did not get your earnings.  They probably use the send account for multiple people and it would be very hard and time intensive to prove that transaction was yours.

Why anyone uses mtgox as their wallet is beyond me, this is why there are ewallets and desktop wallets.

Lesson learned: read the warnings before proceeding with using any system like this.  This wasn't even hidden in a TOS or fine print, it's right there on the front page plain and simple!