Hi,
It pays to read the tournament rules and conditions prior to playing. It normally says something like - the top 70 in the tournament divide up the credit pool.
Once you've finished your round, it will also tell you what position you are currently in. That way you'll know whether you are in the hunt for credits. You'll have to wait till the next month to see what your final position in the tourney was and whether you'll get any credits.
Hope that helps.
Cheers Doug