Edit: the following info is outdated. It still works, but see the verified answer to import directly.
Currently I don't believe you can import into Metamask, so I did it the other way around. Here is how:
1) Go to the account in Metamask and Export the account you want. This will give you the private key.
2) Create a file on your desktop called boring_tax_stuff.txt
and paste your private key (make sure not to add any spaces or anything. You may have to remove the "0x").
3)From a terminal type geth account import ~/Desktop/boring_tax_stuff.txt
or geth --testnet account import ~/Desktop/boring_tax_stuff.txt
if you want to add them to your Morden wallet (they are separate wallets).
4)You should see it immediately in geth console by typing web3.eth.accounts
Unfortunately your primary account or "coinbase" will not change. To change this, I just changed the timestamp in the filename of the accounts. So:
5)Navigate to ~/Library/Ethereum/keystore
or ~/Library/Ethereum/testnet/keystore
and change the dates so that the account you desire to be 'coinbase' is earliest.
6)Change the contents of boring_tax_stuff.txt
, save it, then delete it.
You can restart geth and type web3.eth.coinbase
to verify everything worked!