Go to Devices -> Device Setup -> Add/Remove, and add a Generic Remote.

Click on the Setup tab and highlight Generic Remote.

The top section is what the MIDI message is, and the bottom section is what it does, and the Control Name is the link between them. In this example, we will set a keyboard C to mute/unmute track 1.

Make sure your MIDI input is set to the MIDI port of your controller.

In the top section, scroll down to Mute1 and click on it. Press the desired key or controller and then click Learn. The MIDI status, channel, etc. will change.

Then, in the bottom section, scroll down to Mute1 and we'll assign it. Device should be VST Mixer, Channel/Category will be Audio01, Value/Action will be Mute, and Flags will be Push Button and Toggle. (Toggle will cause Mute to toggle each time you press the controller, leaving this unchecked will mute only while you hold the key). Press Apply when done, and test it. Pressing the key on your keyboard or controller should mute/unmute the first audio channel.