Source: Workflow: Activate a BlackBerry Device Wirelessly (BlackBerry Forums)

Basically, set up an Account in BlackBerry Manager, that is tied to the user's Exchange account. Then, right click on the account and select "Set Activation Password". Then on the BlackBerry Device open the Enterprise Activation application and enter the email address and activation password.

Options > Advanced Options > Enterprise Activation

Asks for email address: enter their email address

Asks for password: enter the activation password? Or their Exchange password?