External Mixer set up help please...

WARNING! - Super Noob Alert!

Hey all. So I'm trying to refine my set up but I have a couple hurdles I'm struggling with. I've got a Mackie ProFX8v2 mixer I've connected to my computer with USB. I feel like I have things set up right/good, but the current issue I'm trying to figure out is, if I plug in a Mic or guitar I want to hear only what's happening in the computer, but I'm getting that and the audio straight from the Mic. For example, I'm trying to use the Vocoder template to figure how that all works, but with headphones plugged into the mixer, I'm hearing both the vocoder and the straight Mic audio from the mixer itself, but I only want to hear the Vocoder stuff. How do I pull that off? I'm imagining that if I try doing some other vocal stuff or guitar recording I'll suffer the same problem, so, any effects applied to the guitar while playing, will sound wrong because of the clean guitar sound coming through the Mixer.

I hope that makes some kind of sense, and that someone has an idea of how to help out.

Thanks.