Hi,
I have Mach3 on my laptop, Windows 8 64 bit and I can assign a hotkey just fine.
In the pics attached I assigned Input#1 as enabled, emulated and emulated with lower case a. After restarting Mach on the diagnostics screen Input#1 LED operates
on <a> as expected.
I note from the link you posted that it would fail with a real time signal like Probe, haven't tried it yet.
Craig