MyBB Community Forums

if you don't have a .htaccess file in your forum folder, create it (either by using the example htaccess.txt of mybb or by creating an empty file). if there's a rewrite rule of drupal interfering this will disable it for the forum and its subfolders

This is all that is needed:

Add this line to the beginning of the .htaccess file in your subdirectory you are trying to protect (mybb):

ErrorDocument 401 "Unauthorized"

From here: http://drupal.org/node/64708