Being forum active means:

-Checking on your clan thread.
-Checking the forum in general
-Posting and ingage "conversation" or interaction with other users.
-Post a few times a day or at least once a day.

Your post count will not go up if its just a clan thread. It must be on the forum itself. It'll be awhile before you reach that mark. Like I said, you may apply later on (not today) once your count goes up. Just being on every day and talking with people is being "forum active". But it won't show unless you visit other threads like Off-Topic, General, News, Beginner Sanctuary, etc.

I'm sure there is other clans out there who are more in game clan than forum. They would love to have you as a member.