FileVault2: how do I use the recovery key when asked for a "Disk Password"?

I've got a MacBook Pro protected with FileVault 2 (10.9), and can't remember the password to unlock the disk. When I boot, it asks for the "Disk Password". What I have is the recovery key that I printed on paper when I activated FileVault. How do I use it ? Entering a wrong password multiple times doesn't prompt for the recovery key, and sometimes a '?' appears but it doesn't do anything. (I've read http://support.apple.com/kb/ht4790 , but it doesn't explain how to use the recovery key.)