cPanel uses system quotas to display what you're seeing, after verifying 
that your kernel has quota support built into it (ask your Data center 
or OS vendor/community about how to do that):

1) make sure the proper partitions are mounted with quota support:

 mount

This should look like this (IE notice the usrquota):

 /dev/hda6 on / type ext3 (rw,usrquota)
 none on /proc type proc (rw)
 /dev/hda1 on /boot type ext3 (rw)
 none on /dev/pts type devpts (rw,gid=5,mode=620)
 /dev/hda5 on /home type ext3 (rw,usrquota)
 none on /dev/shm type tmpfs (rw)
 /dev/hda2 on /usr type ext3 (rw,usrquota)
 /dev/hda7 on /var type ext3 (rw,usrquota)
 
2) once #1 is ok, make sure quotas are on and able to be started:

 quotaon -a

you may need to turn them off first:

 quotaoff -a

3) once #2 is ok, check the system quotas:

 quotacheck -acfmv

4) If that all looks ok and the quotas are all still 0 or you are otherwise
getting an error:

 /scripts/fixquotas
--
Kyle Pinkley
Technical Support
cPanel