HOla, to enable Disk Quotas in plesk, must enter
/etc/fstab
edit that file and add usrquota just after defaults
Example:
LABEL=/home /home ext3 defaults,usrquota 1 2
And then execute this:
quotaoff -av
quotacheck -avum
quotaon -av
You must restart the server end.
SOURCE: http://forum.parallels.com/showthread.php?72591-Hard-Disk-Quota-is-not-supported