"once you have made 5 or more posts this message will be removed and you can post as normal"
Just make 5 or more posts on the forum, if you're in the jam, you'll knock that out quick with updates of your jam game.
Edit: lmao Read it wrong
It will go away after a while or so
As far as I'm aware, your profile's post counter should be what counts for this limit (and it only counts posts in forums where posts... well, count). There's a chance the threshold for this was changed and the message wasn't updated... so something is wrong here either way, and I suggest getting in touch with an administrator (@Nocturne, @rmanthorp) so they can check it out.
You don't have to, and in fact can not, get this off your account on your own. It should be automatic.
Status updated don't count towards the five posts, as they were used for spam just as much as actual forum posts before we instated this rule... and they can contain links and images too. Looking at your account just now and you should have made the required 5 posts, so if you're still seeing this message, please let me know.
EDIT: Sorry, misread your OP. Okay, I'll look into it. I suspect the GMCJam topic isn't being included either since the GMCJam forum was added after this rule was created.