You can access it here: https://forums.macrumors.com/forums/132/ If the forum says you don't have permissions for it, give it a few hours for the servers to make the changes to allow you to access it.
I see this question asked over and over again when a user hits 250 posts. Almost makes me want to suggest a sticky called "I just now hit 250 posts, why can't I access the Marketplace yet?".
I'm looking forward to accessing the marketplace eventually. 250 posts is a lot though. I've been an active member for 6 months now, and try to join in discussions regularly. Still, as someone who doesn't want to just post for no reason it takes a while to get there!