If the user account has been created with a mailbox (mailbox enabled) you may follow ameetsaha's instructions, but if you need to create an email account (mailbox) you will have to go to your Active directory using your Adminpack with exchange componets installed and .

1. Right click the user account and click on exchange tasks and click next on the exchange wizard window.

2. Select add mailbox and click next follow the prompts and direct the mailbox to a mail storage click next after every prompt and finish when done.

Then all Ameetsaha instructed is correct in my opinion.