Just in case:
Unlike some other directories, ODP does allow multiple editors in a category. The question (for metas reviewing your application) is not "is this real estate already staked out?" but "does this applicant have something to offer here?" If you can find three good, relevant websites not already listed, and you think you might be able to find a dozen more within the next 6 months, then you have something to offer! And the presence of one, two, or three current editors will not even slow your application down. (Remember, one or more of those editors might be almost completely dormant, or very busy in other categories anyway!)
So: How to locate categories that need YOU as editor? ANY subject for which you know a dozen websites not already listed, DESPERATELY needs YOU. Any subject for which you know three websites not already listed, probably will be considered to be needing YOU.