Branding is a tough uphill road. You need to do all your seo in concert. Consider the Exactseek.com adword program, $12 per ad word unlimited clicks for three months, the trick is a phrase like quhl web directory will display ads for quhl web directory, quhl web, quhl directory, web directory, quhl, web, directory, if the individual words haven't been fully subscribed.
If your main categories look like links, you could try submitting them to the big three like www.quhl.com/Health-Medicine/
especially if you have enough listings for a niche directory within. Niche directories appear in key word search listings all the time.
See if you can figure out a way to have individual meta descriptions for your main categories the big three can pickup. Like building individual pages of a site as stand alone quality, each page could be a site on it's own.
People don't search for directories (except to add their url or add the dir url to their auto submitter software) they search for what they want and if the return includes a directory in the results with a description in the category saying what they're looking for, maybe they'll click on it.