you should try to update ispconfig installation so it set up you services, prolly you installed squirrelmail after ispconfig3 or something messed up during installation. do it by typing:
cd /tmp
wget
http://www.ispconfig.org/downloads/I...-stable.tar.gz
tar xfz ISPConfig-3-stable.tar.gz
cd ispconfig3_install/install/
php -q update.php
that will updaet ispconfig3 and set-up services