If you're on 6.31, you're looking at the wrong mirc directory
your mirc.ini and user configuration settings are all stored in your Application Data directory
type //run $mircdir from within mIRC or go to C:\Documents and Settings\YOURUSERNAME\Application Data\mIRC
or C:\Users\YOURUSERNAME\AppData\Local\mIRC\ if you're on Vista
Note that //run $mircdir is the easiest method since the Application Data directory is hidden by default