I figured this out:
1) in the "Edit" window for the custom object resource, click "Select Orion Object"
2) in the "Select a Network Object" pop-up window, change the "show only" menu to "Interfaces". This is the key point I was missing before.
3) Select the interface from the right side pane. This is a horrible UI choice here, since I have only a moderate sized installation and there are 75 pages of interfaces to scroll through!
4) Click "Select Orion Object" button.
5) Change the "Select Object Resource" menu to "Min/Max/Average bps - Today"