You have to do it in the Geck, and you have to launch the Geck with the FOSE loader:
fose_loader.exe -editor
Then find the two scripts 3EFdrgsscript and 3EFdrgnvgscript. One is for night vision and the other is for cloaking
In each one, find the line with IsKeyPressed http://fose.silverlock.org/fose_command_doc.html#IsKeyPressed
Just change the number to the key you want and hit the save button on the script, then save the plugin and you are done.