Can you go into MixEmergency's Preferences, to the Transition panel, and make sure that "Use External Controller Crossfader" is enabled. Then, make sure that the button to the top-left of the crossfader in MixEmergency is enabled. If none of that is working, you may need to open up Serato Video and make sure that the crossfader is enabled there - as if it's not working in Serato Video, the crossfader signal won't be sent through to MixEmergency.