According to the Marketplace Info thread, you need to have more than 10 posts in the forums and be registered for more than a month in order to post an ad.
It seems like you just need to wait a bit longer. If you're pressed for time, you can always look at the list of the commissioners to see what they specialize in, then contact them via PM.
Originally Posted by Admin
In order to post ads in the marketplace, you must now have at least 10 posts on the forums and be a member for at least 30 days. This is where most of the abuse was from.