Hey TC - You have to have 10 posts before you can post a classified ad. All classified ads are read only so you can only respond by private message (PM) which is also a privilege you get after 10 posts. I don't know if you are trying to post an ad or respond to one but either way you need 10 posts.
So just pop in over at the New Users forum and say "Hi" to 8 more people and you're done. Then you can use the classifieds like everybody else.