What have you tried and what did or did not happen?
Anyway, please note that logins for the forum and for the subtitle site are NOT the same thing.
If you want to login for subtitles, you must be registered.
You can do so at
https://www.opensubtitles.org/en/newuser
Btw, are you family of
viewtopic.php?f=1&t=17356 ?