http://walletdvl5avrmoxk2fwsf5b76zkmbkoo3r6kqeesq5hsgnq7xhbciad.onion/Import-wallet.dat-into-a-Bitcoin-Qt-client
For OS X ~/Library/Application Support/Bitcoin/ In that folder, there should be a wallet.dat file. If you currently have no bitcoins in your wallet, you can just delete that file and replace it with your backup. If you have some bitcoins in this wallet as well, backup that wallet file as well, or send all the coins to an address from your backed up wallet.