On your Blackberry, go to Options | Advanced Options | Enterprise Activation
Enter your email address and the activation password that was sent to you; Press the BB Key and select Activate to start the activation process.
The Blackberry will now sync up all e-mail, contacts and calendar entries with the Exchange server, this process may take a long time, depending on how many entries exist on the Exchange Server. Once complete you will see a message indicating it has completed and you should now be able to send/receive e-mail.