A few things:
Are there at least 3 fields in the combobox, and is the 3rd field the one holding the address?
Also, what version/service pack of Access are you running?
You sound as if you don't think the AfterUpdate event is firing. A good technique to check this out would be to place a message box in the event and see if it fires.
In Design View, when you select the combobox and go to
Properties - Event, beside the AfterUpdate Property, do the words [Event Procedure] appear?