In the main menu on the left bar, there is a "submit" link just under the Classifieds entry. Really subtle, and that's what I don't like about it. There should be a nice "submit new ad" button at the top of the page, just like there is a "new topic" button in the discussion forums at the top of pretty much every page.
There are other layout issues I am not happy with, but they are really low priority. But so few ads are being posted, I want to make it as easy as possible to encourage people to use it.