open the file you downloaded. upload all files to your server using ftp (easyest way)
then open
http://<your server address> /forum/docs/install.txt
or read this (my install file )
quote:
Installation Documentation
------------
First of all, we would like to thank you for chosing myBB as the software to
drive your community forums.
This file outlines the installation method for myBB on both windows and *nix systems.
Installaiton Instructions:
--------------------------
1. Unzip all of the files included in the zip archive downloaded
from the myBB website.
2. Upload all of the files via FTP to your website, place them in
a directory such as forums or forum.
3. Chmod the following files to 666 (ie.. make sure they are writable)
./inc/settings.php
./inc/config.php
4. In your browser, visit the url where you installed the forums with the
addition of /admin/setup.php on the url.
eg. http://www.yoursite.com/yourforums/admin/setup.php
5. Please follow the setup wizard through the steps making sure you pay
attention to what is happening.
6. After the installation is completed, please remove setup.php from your
server and for additional security, chmod config.php back to 755.
NOTE: settings.php must remain writable through the whole time of your
forums being active, this is where settings are written to.
7. Pat yourself on the back, you should now have a working copy of myBB
installed.
/ 
