Thanks for the response.
I read the help pages you referenced, which really doesn't explain the interactive tool in ArcGis. There are different parameters which go into the Merge Field which controls which features are merged, but the interactive pulldown menu is empty. The pulldown menu should have items in it that can be selected. With nothing there, there is no way to input something into this field, at least I haven't figured out how. I try typing things in there and they are all marked as invalid.
I think the only valid way of inputing something into the field is via the pulldown menu, which is empty.