As you know, the M-Audio Keyrig 49 is a USB keyboard controller.

It serves the same purpose a MIDI controller would in a hardware set-up.  As such, the keyboard controller makes NO sound on its own.  A USB controller is used to control software, or VSTi, instruments.

In MMM, the software instruments would be any of the instruments in the Vita MIDI editor or, in some "premium" versions of MMM, the Revolta 2 synth.  You can also use it to play any VSTi plug-in instruments that you've added to the program.

The M-Audio Keyrig 49 controller is simply connected to the computer with a USB cable.  Make sure you've loaded any drivers that came with the unit.  Verify that your computer "sees" the unit by checking your hardware/device manager (in Windows).

Then, make sure the Keyrig 49 is the chosen recording input device in both Windows (Control Panel > Sounds and Audio Devices) and in the program settings and recording pop-up in MMM.

That *should* do it.