fn key sending

The "Fn" key doesn't need any additional software. Maybe you misunderstand how to use it?

The "Fn" key does not produce any output to the screen, nor does it visibly do anything on it's own.
First you press the "Fn" key and release it, then you press whichever dual-function key you wish to use.

When done, press the "Fn" key again and all the dual-function keys will revert to their alternative function.