You will not see the Marketplace forum until you qualify to use it. To qualify, you need a history of 250 posts (user title 6502 or higher) and membership for at least 6 months. See Why can't I post? for further details.
Once you qualify, it will appear in the Mac Community section.
If you have enabled the "Receive an alert when you receive a new trophy" setting in your Alert Preferences then the forum system will send you an alert when you are first eligible for the Marketplace forum.