In Showthread.php

Look for:
$userpostcount=$userinfo[posts];

After this put:
$days = unixtojd() - unixtojd($userinfo[joindate])
$userexp = round(pow(((pow($userpostcount,3))*($days)),0.5));
$userlevel = round(pow($userexp,(1 / 3.5)));

To show it in the templates open up the postbit template and put $userexp, $userlevel where you want them to show up at.

p.s. I think I got parentheses happy.