It's very easy to drop the combo box down in its Enter event:
Me.DropDown
If you want to respond to typing, use its Change event. You would need to
know whether the combo is already dropped down or not. Access doesn't
actually give you this, but it is possible to get it with an API call:
Determining Combobox's Dropped state
at: http://www.mvps.org/access/api/api0052.htm