I'm not entirely sure of what your question is, but I believe it's just a setting in the reseller section. You should be able to add domains and define quotas if the options are set correctly.
Try this - (if you have root on the box) login and set 'all' priveleges for the reseller. Then login as the reseller and see if it works. If it does, then it's just a matter of setting the right priveleges in the reseller setup!