This will happen if you install NuSoap 0.7.3 instead of 0.7.2 (as specified in the instructions).

Roll back the NuSoap version and it should work... EXCEPT that if your server is running PHP5, you'll get errors from NuSoap 0.7.2 like this:
Fatal error: Cannot redeclare class soapclient in /path/to/nusoap.php on line 7240

That's because PHP5 has it's OWN built-in SOAP client that uses the same name.
To fix this, you have to either run PHP4 instead, or edit the NuSoap code a little bit.

Search Google for the error to find instructions.