Edit: the following info is outdated.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
Unfortunately your primary account or "coinbase" will not change. To change this, I just changed the timestamp in the filename of the accounts. So:
~/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!