BobbyMitel,
I am no 3300 guru, but it seems to me that all you need to have them do is go into Users and Services Configuration and under that SIP device on the Service Details tab set their DID number in there and then check the box to use their DID for Outgoing calls and that should fix that problem.
The Associated Directory Numbers form is if the DID doesn't go directly to that device, but you want it to show that DID when making outbound calls; either way should work fine.
Thanks,
TE