It looks like its not supported by minibb 3.0 ? Or did i make a mistake ?
This add-on should work with 3.0 and any other version, in general. It doesn't contain anything specific, so most possibly there is a hiss on your own end.
I've noticed in the manual that there was a member ranking system.
Please point us to the exact point of Manual where did you read that. There could be assigned various member titles according to their posts number, as it's described in Custom member titles
. It is only about very special members and it's not automated (my title on this forum is assigned using this approach).
Custom Rankings add-on is more about automated assignments.