If I m not wrong .. there is a executable file conf.pl , in squirrelmail dir, just make changes in  that file..using command
#./conf.pl
There are number of optons there
U have to give ur  domainname,sendmail server and imap server.., U can give that in corresponding options..
Before giving  imap server, be sure it must be started on ur server
#/etc/xinetd.d/imap 
change disable=yes to disable=no 
atart the service by: 
#service xinetd restart 
After that go to your web browser ,then type url as 
127.0.0.1/webmail 
or 
localhost.localdomain/webmail 
or 
domainname/webmail 
{ 
where domainname is your Domain......} 
but before using squirrelmail , ensure that ur named ,httpd,sendmail must be restarted} 
Gaurav