I believe the extra credit option in Gradebook 1.9 does not work at this time. I recall earlier discussion about this and think it was added to the list of issues to be resolved.
We work around the problem by entering a formula in the category calculation, but it gets a bit messy. For example, in the case where a faculty member had three exams (w, e, a), each with three components (hands-on, written, extra credit) our calculation formula is :
where who = exam w, hands-on part; wob = exam w, objective part; wex = exam w, extra credit part, etc. We also had to change the category maximum grade to 110.
It's a pain to work with because the faculty member always needs my assistance when he adds another exam.