If you're talking about the TCX to Nike+ uploader here, http://www.awsmithson.com/tcx2nikeplus/, I don't think the TCX upload option works any more (Nike changed something on their site to break it?). At least, it never has for me. However, uploading a run via Garmin Connect ID does work, so the guide that someone else posted earlier on is the one to follow -- get your run into Garmin Connect (e.g. via uploading GPX), make the run public (flip the padlock, you can always lock it again after importing to Nike+), grab the ID from the URL, then import to Nike+ via tcx2nikeplus.
It's a shame that Nike+ it such a walled garden as it's got some nice shrubbery. Import and export of GPX should be essential. I tend to run the Nike+ Running app alongside My Tracks on my phone now, so that the run goes into Nike+ and I get feedback, etc., but also have a GPX track (high-res, 1 second too, which is better than most running apps) from My Tracks to import to Strava and Endomondo.
P.S. FWIW, it is also possible to export GPX tracks from Nike+. There is a Chrome extension (not in Chrome web store, search and install manually) that does it, but it's a slight fudge - even though position data for all trackpoints is accessible, apparently the timestamps are only available for the km markers, so times for all other trackpoints in between are synthesized from the km points.