Easy. Change read_pms.php name to (for ex.) super_admin.php, open read_pms.php (or super_admin.php if already changed) and where its says:
define('THIS_SCRIPT', 'read_pms'); change it to:
define('THIS_SCRIPT', 'super_admin'); open cpnav_rpm.xml and change this lines to what ever modified above:
Code:
<navoption displayorder="130">
<text>Read PMs</text>
<link>read_pms.php</link>
</navoption>