Since this is a Pre-Alpha version, I encourage you to make vanilla WordPress and MyBB installs to test the plugin on your server first, without screwing up your website and message board.
WP side
To install WP/MyBB just extract the "wp" files in your WordPress folder and the "mybb" files in your MyBB folder.
Then just activate the WP plugin from your WP admin section and configure it going to Plugins -> WP/MyBB Configuration.
You can use the following WP/MyBB functions in your theme:
- wpmybb_get_thread_link()
- wpmybb_comments_popup_link($line)
wpmybb_get_thread_link() returns the URL of the corresponding thread of the current post (if it has one) in the WordPress Loop.
wpmybb_comments_popup_link() is the same as wp_comments_popup_link, except that it will show a link to the corresponding thread instead of the WP comments link if Comments on WP is set to false in WP/MyBB Configuration. Plus it will display the username of the last poster in that thread. Just replace wp_comments_popup_link with wpmybb_comments_popup_link in your theme, and you're done.
You can customize the "Last comment by $user" line passing a string to wpmybb_comments_popup_link(). The $user tag will be replaced with the username of the last poster.
Example: wpmybb_comments_popup_link("Last post by $user");
You should also delete from your theme the HTML code that displays the "Leave a reply" box, since the WP comments system won't work.
MyBB side
If you want that adding, editing or deleting MyBB posts in the specified forum section will result in adding, changing or deleting WP posts, you have to activate the MyBB plugin and configure it from the Admin section of your message board.