Joe,
Are you trying to upload running workouts to Nike+, or just your walking steps? It used to be that your walking steps would upload to the Nike+ Active website, but they shut that down almost 2 years ago. Running workouts would upload to Nike+ Running. For a short while, the Nike+ Running website would accept the walking steps, but no longer. I've given up tracking my walking steps via Nike.
As for the file that stores your Nike id info ... read on. You're running on a Mac, right? If you're using a PC, it's a little easier. Just drill down to the same folder as described below ... you may have to "enable invisible folders/files" or something like that.
1. Connect your iPod to your Mac.
2. Go to the Finder and hit Shift-Command-G
3. Type in /volumes/*****/ipod_control/device/trainer/workouts/empeds , where ***** is the name of your ipod
4. You'll be taken to the empeds folder where you'll see a folder for nikeinternal and pedometer. nikeinternal is for your running workouts while pedometer is for your walking steps.
5. Your Nike+ credentials are stored in the settings.plist file. It's a text file, so you can open it in TextEdit (or Notepad for Windows users).
Hope this helps