Very odd message. I also run Windows 7 64 bit and it works alright.
A few things that might be worth trying:
1) Try re-downloading the GRSS mod first. See if that helps at all. Or redownload just the GetGRSSDKP program (
http://dkpfiles.com/www/forum/26989/GetGRSSDKP.exe )
2) If that doesn't help. Try moving the World of warcraft folder out of the "Program Files" folder. It shouldn't be related, but the program files folder is protected by Windows and programs can act finicky in there. I keep my copy of WoW in c:\Games\WoW (and I put all games in C:\Games). Doing so prevents any silliness revolving around the protection schemes (and will make finding things like screenshots and data files easier).
If neither of those help, let me know, and I can set up a remote support thing.