If you are following the tutorial, you have already set up your Git repositories to use SSH. This page has you set up your Mercurial repositories to use SSH as well. If you skipped the Git instructions, you missed the explanation of basic SSH concepts.

Setting up an SSH identity can be prone to error. Allow yourself some time, perhaps as much as an hour depending on your experience, to complete this page. If you run into issues, check out Troubleshoot SSH Issues for extra information that may help you along. You can even skip this whole page and continue to use HTTPS if you want.