You want to connect your forum login with a full website login you mean?
I know it's fairly simple to do with a PhpBB forum, however i've heard more issues when trying to combine a website login with the MyBB login. The cookies essentially have to be able to transfer to the login on the site and the forum.
Try taking a look at this:
http://community.mybb.com/thread-44520.html
You could try to install them on the same database with different table prefixes and also in each of the login/registration procedures also performed.
You could try using a Content Management System and a bridge..
ok thanks I will look at it and take your advice