Enterprise Activation (aka Wireless Activation) has the following 4 main stages:
Stage 1 - Activating
Stage 2 - Verifying Encryption
Stage 3 - Receiving Services
Stage 4 - Slow Synchronization
If you are stuck at "Activating...", the flycatcher message (ETP.DAT attachment with the user's PIN info, email address, activation password, BES info etc.) is not processed. Otherwise, it would proceed to process the next stages with these activation information. Hence, you may like to check:-
1. This message ever make to the user's mail box.
2. If it makes to the mailbox, is your BES able to access this mailbox and pick up the ETP message.
3. The user enter the correct email address and activation password on the BlackBerry device.
4. Activation password was set some 48 hours ago and expired. etc....
For more information, RIM has a pretty good and well documented step-by-step troubleshooting steps for you to follow through. Good luck!
KB05202 - How to identify and troubleshoot enterprise activation issues