It's been a while, and I'm not sure if the NEXYS3's PS/2 emulation requires it (I have a suspicion it does, because I recall that my ATLYS doesn't generate any output when a keyboard or mouse is attached and you don't do anything else), but normally you'd send 0xFF 0xF4 to the keyboard to reset it and enable reporting.
If you look at the PS/2 design I linked in my previous post ("PS2 Mouse Control reference component"), you'll see that it does actually initialise the mouse when the controller is reset.