In order to get quota's within Plesk working correctly, you will not only need to download the quota package, but you will need to modify the fstab and run quota.
- Modify /etc/fstab by adding usrquota to the options for example:
/dev/sda1 / ext3 defaults,usrquota 0 0
- Remount the partiton
mount -o remount /
- Turn off quotas
quotaoff -av
- Check quotas
quotacheck -avum
- Turn quotas back on
quotaon -av