I spent far too much time in trying to change the default logo link in MyBB, so in an effort to spare you the same fate, here’s where it is:
Home » Template Sets » Default Templates » Edit Template: header
<div id="header">
<div id="logo">
<div class="wrapper">
<a href="http://www.example.com/"><img src="{$theme['logo']}" alt="Example" title="Example" /></a>
<ul class="menu top_links">