If you are attempting to post in the Member Classified as a Newly Registered member, please understand that access to the MC requires site run time (min 30 days) and post count (min 15 posts) before the site's system grants you access.
That announcement is posted at the head of every MC sub-forum and in the FS rules on the Home Page.
Until you have reached the minimum days and posts threshold, you can see threads in the Member Classified, but you are unable to create threads, or post in another member's thread.
If you're posting in this thread, in this forum, then you are site registered and nothing further needs to be done for you to peruse and use the remaining portions of the public-facing side of the site.