If its the mw3 forum you want, log in on
Even if it says you are logged in, log out and back in there.
You should be able to respond to any of the threads on that page, but in order to start a thread you need to choose one of the sub forums first. Then look on the right side of the page and there should be a heading that says actions and a link to "start
Really, nothing else I can do on my end and with so many people having problems, their forum software obviously has issues. They even had a sticky post which told people to make sure their forum username was filled out in the profile because of this issue,
obviously most people already had this done.