To import just an XML file into a XFA form use the menu Edit > Form Options > Import Data…
We are currently testing Acrobat DC and a user brought this bug to my attention today. What you are looking for is the "More" dropdown button that contains options including Import/Export Data and Clear Form. The "More" button appears in the right-hand pane when editing forms, however, if the form was designed by LiveCycle designer, Acrobat won't let you go to the form editing screen and thus you can't get to the More button.
The workaround, I found, is you can add the More button to the Quick Tools toolbar and just access it that way:
- Right-click an empty area in the toolbar and select "Customize Quick Tools..."
- Expand "Prepare Form" and click on "More Form Options" and click on the plus/up arrow to add it to Quick Tools