Well, you can do:
Me.MyControl.SetFocus
Me.MyControl.DropDown
But after hitting ENTER isn't really a good event to try and do this from. You'd probably have to sacrifice having a normal tab-order and manipulate the right events that would get you back into the control just so you can re-open the list.
A more common way to show additional fields from the selected record is to add unbound fields to your form and then reference those dropdown fields.
Me.cboMyComboBox <-- your dropdown
For each added field just reference the dropdown column number. So if you added (4) fields, the control sources would be:
=[cboMyComboBox].column(1)
=[cboMyComboBox].column(2)
=[cboMyComboBox].column(3)
=[cboMyComboBox].column(4)
And after hitting ENTER you'll now see the related fields. Just edit the column numbers to match what you want to see. Also don't forget that column(0) is your first field.
HTH,
Mike