That link is for router IOS but the commands are preset on recent switch software revs. They're "unsupported" so they don't show up in the switch documentation and therefore I can't tell you what minimum rev you need to be running. I also believe you need to be on a switch family >=3550.
Edit: I just checked my Cat6.5k (SUP2/MSFC2) and the command is not present. My Cat4.5k does have it. Bottom line is YMMV. :-)
Edit2: You also need to specify the file system where the files are that you want to diff. In your case, system:running-config and nvram:startup-config
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