If you have Vista or Windows7, AND you installed your game in the default directory, then you will never see the mod in the data folder.
The GECK is NOT part of the game so the OS sees it as a virus or trojan program trying to change FO3 game files. This is not allowed by the OS.
Solution? Pick "one" of these:
1. Install your game to your Root C: drive. Like C:\Games\Fallout3\ or another drive entirely if you have one.
2. Remove the security from your OS and then you can join the XP computer ranks and become a Zombie computer. *Not recommended.
3. In your \Fallout3\Data\ folder, look at the top right. There should be a button called 'Compatibility Files'. Click that to find your mod. You can then copy it from there to your Data folder.
4. Run the GECK in Administrator mode. Right-click on the icon and select, Run as Administrator.