This error occures when W3GPC can't find Warcraft III on your system.
Copy W3GPC in your Warcraft 3 main folder and run it there. DONE
If the error stays, contact me!
Otherwise you can now move W3GPC into another directory without getting the error again, the path is safely stored in the registry.
Write the required registry keys urself. This is done by following one of the guides underneath.
Write in the command line regedit to open the registry editor.
First navigate to 'HKEY_CURRENT_USER\Software\Blizzard Entertainment\Warcraft III'.
Then use on that key rightlick -> new -> string value and name it InstallPath. Afterwards open the string via rightclick -> modify and insert the path of your Warcraft III, for example: C:\Program Files (x86)\Warcraft III
If you prefer a video guide, check this out, it's exactly the same:
http://www.youtube.com/watch?v=cVzIzBFtVXo