I couldn't find the option, I clicked on every menu. What menu is it under?
But I found this:
http://support.coinbase.com/customer/portal/questions/695389-backup
http://blog.coinbase.com/post/33197656699/coinbase-now-storing-87-of-customer-funds-offline
Unless that is out of date, they don't just leave the funds in any of the addresses you see in your account, so it would be futile to try getting the private keys of any of your addresses anyway. I can see advantages to doing it that way, but you have to trust coinbase like you would a bank, and I'd prefer to be responsible for my own coins. They are by far the best for buying and selling bitcoins, but I will always send my coins to one of my own wallets right after.