Hi Baz,
If you are in the first 4 channels (mic channel), then the pan control should take the signal from hard left to hard right in both the main outs and the control room outs.
If you are in a stereo channel, then the pan will shut off one side to favor the other. If a stereo signal is plugged into a stereo channel, then the pan should be set to center.
The Zenyx 802 has NO effects built in, and the FX knob will not produce a sonic change unless you hook up an external effects unit, like a reverb or delay.
heres a youtube video of a typical setup:


I don't know what would cause squealing, unless you have high gain and poor speaker placement. Call me if you need to ... I'll be happy to talk you through it.215-284-1328
Every instrument, every voice, every sound in nature ... these are are all my private tutors ... 24/7 for free, and all I have to do is listen.