Is this BES 3.6 or BES 4? What mail system do you use?
Proceedure should be as follows. This part is the same for BES 3.6 and 4.0:
1. Add user to BES. Status should be 'initialising' with a pin of 0.
2. Open desktop manager. It should say 'online' down the bottom.
3. Plug in BB to desktop. On the manager, click tools/connection settings. Select USB and click detect, then OK twice.
4. Manager should now show 'this device is not associated with this user, do you wish to associate it?' Click Yes. It will ask to generate a security key, click ok and move the mouse around till it finishes.
At this point, on the BES your user should now have the PIN of the device associated with your user. Now you can synch your contacts/calendar over ect ect.
BES Admin, sometime God.