In order to monitor client disk space usage in Plesk, disk quota must be enabled. This is a two step process and the initial quota scan may take some time (~1 hour) and may cause notable load on your server so I recommend running this outside of your peak load times.
1. Add userquota to the FSTAB options:
/dev/md0 / ext3 defaults 1 1
FSTAB with usrquota enabled:
/dev/md0 / ext3 defaults,usrquota 1 1
Lastly, remount your file system to enable quota support then run a quota scan:
# mount -o remount /
# quotaoff -av
# quotacheck -avum
# quotaon -av
More information regarding disk quota options can be found here.