Which version of SBS, as o0HoeCool0o is giving you instructions for exchange 2007
If you have SBS2003. You'll need to go to active directory users and computers, click view menu, tick advanced features.
Go to the user that you want to send as. Right click, go to properties. Go to security tab, add user who needs permission. Scroll down the list you'll see Send As, click allow. Wait for propogation, as sire_harvey says, it will take a while.
