Your forum should be in a folder called Forum in your server root right?
And also, go to admin cp >> Config >> General config
and alter your board url to
http://site.com/forum
Your URL where, on that page under General Config?
Just make sure that if you want to have your forum to be at site.com/forum that you should:
a) install your forum into public_html >> Forum on your server
b) update your Board Url on the General config page so myBB knows where it is too.
You do not need to install again as the previous poster would have you believe.
If you have already copied all the files from /forum to / then go ahead and delete the /forum folder, login to your admincp >> Configuration >> General Settings >> Board URL and update the board url there with your current url which should be
http://www.urlhere.com
Also check your cookie path, it should be set as /
Save the changes and everything should now be good.
If you can not stay logged in to make the above changes then you will first need to make these changes manually in inc/settings.php file and then once you can login make the above changes.