Go to phpMyAdmin, find the user_id from the wp_users table (let’s say it’s 30), then select all metas from wp_usermeta:
SELECT * FROM wp_usermeta WHERE user_id = 30
There you should see a record with meta_key = wp_capabilities
Edit this record and change bbp_moderator to bbp_keymaster, leaving everything else intact. (Assuming you assigned yourself a Moderator role beforehand).
In may case I had to replace this:
a:2:{s:13:"administrator";b:1;s:13:"bbp_moderator";b:1;}
with this:
a:2:{s:13:"administrator";b:1;s:13:"bbp_keymaster";b:1;}