In order to access more than one instance of a repeating node, you must use a repeating control of some kind.
For your scenario it sounds like the best choice would be to use a repeating section. Create a Repeating Section without controls, or just delete all of the controls out of it after it is inserted. Then add just the attrbite field that you are interested in.
You can remove the white border, padding and margins on the section so that it doesn't take up more space than is needed.
You can also specify whether or not the form user can insert/remove items in the Repeating Section Properties dialog box.
Greg Collins [InfoPath MVP]