I've had the same problem. The issue is the KARAOKE button at the bottom of the effects rack simply does nothing at all. In my MM-10 it works just fine.
In MM-15, when I've tried to use the FX Plug-In (DirectX) CT Karaoke Filter, I get the following error message:
Type conflict! The causing Plug-In(s) CT Karaoke filter,...will be removed!
I have the same problem as Procyon. That actually goes for most, if not all, of my DirectX effects. It's pretty annoying. Anyone figure out how to solve it yet?