Here's an article I wrote on how I installed Roundcube on my Plesk server. Comments/suggestions are welcome.
http://www.gregbeach.ca/how-tos/463-install-roundcube-in-plesk-11-and-centos-6-x.html
http://www.gregbeach.ca/how-tos/463-install-roundcube-in-plesk-11-and-centos-6-x.html