MyBB Community Forums

Yes I have read the important thread about this, I just have a question.

Which one do I update?
Board URL or Homepage URL

Basically the problem was when I try to login to my forum it just takes me to the main page without logging me in.

Please provide the URL for your forum, and your settings.php file (/inc/settings.php)

It sounds like a cookie problem
Update
$settings['cookiedomain'] = ".example.com";
$settings['cookiepath'] = "/";
$settings['cookieprefix'] = "";

With a cookie prefix of your choice (doesn't matter what it is, this will help it from conflicting with other applications you may run) and update the cookie domain to .yourdomain.com