Squirrelmail (Webmail application) can be installed within the control panel under:
Home >> Hosted domains >> $DOMAIN >> Web Applications >> Install >> Squirrelmail >> Next
The settings you need to apply are as per the screenshot below. (Hosted Blacknight mail)
Take note of the editing of the "Trash, sent and draft" paths.
Also replace the IMAP and SMTP mail server details with those of your email provider.
Should you have any queries, please contact firstname.lastname@example.org