In Plesk 7.1.x, you could simply pop it into /home/httpd/vhosts/webmail/httpdocs and it would like a champ. I'm running Plesk 7.5.3, and this is no longer the case. With that said, it's still possible to get it working...
You'll need to download the latest Squrrelmail from their site (v1.4.4 I think) and expand it somewhere on your machine (say /usr/local/webmail/squirrelmail-1.4.4). Lock down permissions on the file as restrictive as you can get away with (see the install guide for SM for more info). Also create a directory for data and attachments (these would be writable by the WebServer), so you could use (/usr/local/webmail/squirrelmail-tmp/data and attach). Set permissions on those properly too.
Once you have squirrelmail out there, use the configure script to set your Squirrelmail installation up. See the SM docs for that too, it's pretty easy.
Now edit your httpd.include file (think it's /etc/httpd/conf/httpd.include) and look for the 'webmail' Virtual Hosts on 80 and 443. You'll see an alias in each one for /horde and /imp... Just create a new alias called squirrelmail (or whatever you want), and configure it to point at the path you deployed to above.
Once all that is done, restart httpd (/etc/init.d/httpd restart) and try hitting squirrelmail...
That's all off the top, so please forgive any typo'd paths. If you're having issues, I can fill in the gaps when I get back to the house (and my machine).
Joe