Simply put, you just have to be patient. Ive mined since day one and I didn't receive my first stake until earlier today. If you have encrypted your wallet then follow the directions in this thread to "unlock" your wallet. If you HAVEN'T encrypted your wallet then just make sure you're running the latest version and wait. The chances of you obtaining a stake increases with the amount of coins in your wallet if I understand correctly. Just make sure to keep your wallet open and you should get a stake eventually!