Host bitbucket.org
IdentityFile
~/.ssh/<em>privatekeyfile</em>
The second line is indented. That indentation (a single space) is important, so make sure you include it. The second line is the location of your private key file. If you are following along with these instructions, your file is here:
~/.ssh/id_rsa
When you are done editing, your configuration looks similar to the following:
Host bitbucket.org IdentityFile ~/.ssh/id_rsa