There is a known bug when logging on via the forums where it will display the sign up sign in option but clicking the buttons won't do anything.
This is caused by a syncing issue between the forums and the marketplace and is currently being worked on.
In the meantime, if you want to log in, try logging out from everywhere first (including the marketplace), then logging back in.
This should fix the issue for most and you should now be logged in on both sides.
Thanks for your patience while we resolve this issue!