If your directory has sites/software etc. separated into categories, then it can be listed here. Otherwise if its just a bunch of links all on one page, it should be submitted to Arts/Music/Sound_Files/MP3/Link_Lists/.
Please also note that this category DOES NOT list sites offering the download of MP3 files (i.e. download directories). These sort of sites should be submitted to Sound_Files/MP3/Downloads/.
This category is used for listing MP3 Directory sites. These are sites on the net that list other MP3 sites/software/skins in categories usually with a search function so you can find just the right site you're looking for.
You can usually add your site to a directory by going through a procedure explained there, or if you have an MP3 Directory site, you can list it here by going back to the category page and clicking "add URL".