Try using a Device Key trigger to see the eject key press.
The Fn key works as a hardware toggle that allows a single physical key (eg F1) to act as two distinct keys (eg F1 and Brightness Down). The Fn key is not a modifier (like Shift or Control). It does not apply to many keys other than the function keys, and cannot be used as part of a hot key or device key trigger. You could presumably use Control-Eject or Shift-Eject or something along those lines, but not Fn-Eject (unless it happens that the Fn-Eject key actually produces a different keystroke, which is possible, it depends on the keyboard).
I reactivate this thread because I remapped the eject key to forward delete on my Magic Keyboard a long time ago. But with Keyboard Maestro 8, it does not work anymore. It looks like my keyboard is no longer recognized by Keyboard Maestro.