First off, I'd not use the RPM, it's just as easy to grab the squirrelmail tarball and untar into the directory where you want it to live, just doesn't make sense to have an RPM for such a thing?
Second, if you do install it, it will likely place the directory containing squirrelmail config files et al in a location that RH uses as the default location for it's webserver. Either way a:
Should be able to show you where it went.
Then you'll need to configure it:
And be sure you have an IMAP server running, and a mail server. RH I believe by default uses UW-IMAP.