Just to be clear, what do you mean when you talk about import? As John suggests, if the .xmp file is in the same folder as the .nef file, the parametric changes (edits) should show up when the file is opened again in the ACR editor. But, did you save the original changes? Or did you possibly save the .nef file as a copy and you are trying to open the original?
Or are you talking about importing the file into a new catalog or opening from a memory card? Or are you using some other workflow?