I have submitted 6 problem set from 0-5, but only the problem set 0 haven't been graded. Who should I contact to inform the ungraded problem set?
Go ahead and email Rob Bowden, he handles this sort of stuff.
I think the first one is more convenient as it is his cs50 email and that would be the place he checks for these sort of emails. However, his reply might come anytime from 20 minutes, to 4 or 5 days. You might have to give it that little bit of time.
Hope you get the help you need!