Lets see, I have used Mandriva/Mandrake on all of my mail servers. I can tell you that the directory for squirrelmail was found in /var/www/html/<whatever I named the directory>
When you access your webmail, take a look at the address you type in. Squirrelmail runs under your web directory, so traverse to there, and then move deeper into the directory structure based on the address you type in your browser (i.e.
http://MoMule.is.great.com/mule/webmail - I would go into /var/www/html/mule/webmail to find my webmail files.
Hope this gets you started,
Deion "Mule" Christopher