There are a few things you must ensure to get the effect processor to work. First, make sure the channels effect send knob is set to an acceptable degree (preferably in the middle). Secondly,

the effect drive knob on the processor should also be set to allow the audio to be heard. Next, ensure the level control of the microphones channel is also turned up. Finally, the effect processor must be turned on (certain models may require a footswitch for this to be accomplished). Provided all these points are adhered to, you should be able to select and use any of the mixer's built-in effects.