There is an error in NX default schema used to import run. As, when I try to import runs using the nx default example (smaplepiperun.xml), I am getting error that Cant read the attached xml file, check that correct schema is used.
Download the attached file; SamplePipeRuns.txt, and rename the file extension to .xml. Place it in a directory with a copy of your PipeRuns.xsd (schema) and choose Import Runs. It should work now. There was an unrecognized attribute and a value missing.