Just add the following lines to your root .htaccess file (directly after RewriteEngine On):
Code:
<pre class="_prettyXprint">
RewriteCond %{IS_SUBREQ} false
RewriteRule ^index.php$ http://www.example.org [R=301,L]
</pre>
Of course you have to replace http://www.example.org with your own MyBB installation.