Keith,
I just took a look at the S12 (almost an exact copy of the Mackie 1202VLZ)

You have 4 stereo inputs(ch. 5/6, 7/8, 9/10 and 11/12). You have to plug in TWO 6.3 mm jacks (TS of TRS), left and right channel per stereo input.

In fact you have 2 more stereo-inputs : AUX1 return and AUX2 return. You could use one or both.

The Virualizer can best be connected to the main-out INSERT points, so the signal is processes directly in the audio chain.
You need a TRS (tip, ring, sleeve) here : Tip is send, ring is return, sleeve is shield/ground.

So, you have 6 (!) stereo inputs and 4 mono ones. Ch. 5/6 is switchable between mic (mono) and line (stereo) ?

Hope this helps ?