That is not exactly what "Active topics" does, it just shows the topics where new posts can be found since a certain moment in time (the last time you logged in by default).
So, if there is just one new post since you just logged in, it will show one, if there are 300 it will show 300.
BTW, I think it's included in the base code right now, can't you find an "active topics" link in the forum menu?
Best Regards
Mikel Perez
"Hell is the place where everything test perfectly, and nothing works"