Some education is essential to enlighten you about this form of interactive marketing. Directories aren't Search Engines; they are web sites or information portals which use analysts on staff to review submissions from companies who want to be listed in their Directory. Yahoo would be penultimate example of this type of a portal. But there are literally thousands of other sites that group and list web sites based on their analysis criteria. Here are some tips garnered from years of providing these services to our clients.
1)How do you find Directories? I don't have an easy solution for you, as we've spent years developing and refining (ongoing) our own list of Directories. But, look around on Internet using Search Engines to find popular sites that list resources in your vertical market segment.
2)Directory listings are a great value, as they provide a long-term listing, unlike Search Engine rankings which are much more volatile. Once you achieve a listing you are typically locked into a Directory's database for a very long time.
3)Let's start with twenty-ton gorilla in this market, Yahoo! Spend time carefully reviewing their various categories, then take plunge and use their "Business Express" service which costs just under $200. (USD) to submit your site. Don't, I repeat don't submit unless you have carefully analyzed where you belong in their huge category selections; if you make a mistake you only have one chance to get a new or revised listing, as they aren't very forgiving of those who don't do their homework before submitting.
4)Be patient when submitting to Directories, as most are getting tens or hundreds of thousands of submissions per day – it will take 30-90 days to actually get listed. Keep an accurate record of sites you have posted to and then check back 60-90 days later and resubmit if necessary, but don't spam them, as this will not garner any results.