With the latest update of the site the Classifieds part of the site has now got its own section of the site. The link is now at the top among the other main links for the site.
If you want to post something in the classifieds this is in the same section as posting an image, onto forum, or if you are one of the people with access a story on the Front Page. If you don't know where I will give the instructions below.
Down the right hand side of the page you will see a menu with your Name as the title. In this menu there are a few options. You might know this menu if you ever get privatemsg's via the site. Click on the link labelled "Create Content", then click on the one labelled "Classified Ads" and then simply select which area you want it posted and fill in the information!