This is a bridge between WordPress blogging platform and MyBB message board.
In order to install WordBB just extract the "wordbb" directory in your WordPress plugins folder. Then activate the plugin from your WP admin section, go to Plugins -> WordBB Configuration and set the required fields such as MyBB's root directory and URL.
Once installed, new sections in your admin panel will appear, such as WordBB Posts and WordBB Categories. Also, in the Users section you'll be able to link WP users to existing MyBB users.
WordBB adds some functions which you can use in your WordPress theme:
wordbb_thread_link() : echoes the URL of the thread corresponding to the current post in the WordPress Loop.
wordbb_get_thread_id() : returns the ID for the thread corresponding to the current post in the WordPress Loop. You can also use this function to check if your WP post must use the WP comment system (i.e. show the "Leave a reply box" at the bottom) or just show a link to the thread (something like "Discuss this post on the forums").
wordbb_comments_popup_link([same parameters as wp_comments_popup_link]) : this one is the same as WordPress' comments_popup_link, except that it will show a link to the corresponding thread instead of the WP comments link if "Use MyBB as comment system" is checked and "Show MyBB posts as comments on WordPress" is unchecked in WordBB Configuration. Just replace wp_comments_popup_link with wordbb_comments_popup_link in your theme, and you're done.
wordbb_last_comment_by($line) : echoes a "Last comment by user" line. You can customize it using the $line parameter, in which "$user" will be replaced with the username.
wordbb_get_username() : returns the username for the currently logged in MyBB user.