If you already have a Themetry account, log in at https://themetry.com/account/ and then return here and click the "Log In" button in the top right corner.
If you don't have a Themetry account, sign up first at https://themetry.com/register/ and then return here and click the "Log In" button in the top right corner.
Basically, you must be logged into the main WordPress site at themetry.com before you can login here. This also gives you access to all the free theme downloads without having to manage separate logins for different sites.
Make sure you sign up with a valid email address, or you may be locked out of your community forum account.