Download one of these files (choose based on what architecture is your system - either 32 or 64 bit). Open the file (right-click and Edit should work). You have to change the line that start InstalledPath to reflect the directory YOU have the game in right now. For example, your directory is C:\games\Fallout3. The line should look like this:
"Installed Path"="C:\\games\\Fallout3\\"
Please note you
HAVE to double the backslashes. Then save the file, double-click and confirm adding it to your registry. You should now be able to run FO3Edit without a problem.
http://stashbox.org/v/825036/Fallout3_Installed_X32.reg
or
http://stashbox.org/v/825037/Fallout3_Installed_X64.reg