Are you using the 57 directly with Scratch Live? Or are you still using Serato DJ with the SL 3? If you're using the 57 directly with Scratch Live, check that the crossfader mode is enabled on the Control Window (to the top-left of the crossfader in MixEmergency), and that in the Transition section of the Preferences that "Use External Controller Crossfader" is enabled. If it's still not working, open Video-SL/Serato Video and make sure that the faders are working in this - as if they're not working here, then MixEmergency will not be able to use the faders either.