There are a few things that you need to check.
1. First off if the pin to pin is not going through you need to call your provider and make sure the unit is provisioned.
2. If the pin to pin goes through, check the user's outlook and make sure they can send and receive email. if outlook is OK, go ahead and check to see if you see any emails in their inbox that look like this:
If you see any of these messages in the users inbox, its possible the BES is not picking them up (which would mean a reboot of the server may be needed) OR if you all have a spam filter/blocker, its possible the filter is modifying the subject line, in which case the BES cannot process the etp message. If you do have to reboot the server make sure and remove ALL of the etp messages before trying the activation again.
3. If you dont see any of these messages, make sure that the user does not have any forwarding rules that may be redirecting the etp message elsewhere. Also make sure you dont have a spam filter that is blocking the etp message. if you do, you will either need to disable the filter or allow the etp message
4. Verify you are putting in the correct email address during the activation
5. if you have been resetting the enterprise activation password to the same password, you may want to go ahead and set it to a different password and try the activation again