Can you go into the Advanced section of MixEmergency's Preferences and check that you don't have "Only load from Media Bank" selected.
If it's still not working then I would also suggest checking which pair of channels you have set as your Primary Decks in Serato DJ Pro.
If that doesn't fix the issue, then check that you can load any video at all (by dragging and dropping it from the Finder onto one of MixEmergency's Preview windows).