The trick is that you do not want to use the stereo output on the rear of the mixer to drive the sound for the house. You want to use the monitor output instead, which is a pair of balanced 1/4 inch jacks. In order to get this to be a mono output, you go to the DIO/Setup screen and select the Monitor tab there. In the bottom right corner of the screen, there's a switch that lets you select a mono output. (This is a good thing, because this way both sides of the house will hear the same mix, even if you're panning channels left and right to capture a stereo mix on a recorder.)
Now that you're running your house sound from the monitor jacks, you can adjust the levels quickly by dialing the Monitor Out knob at the top right of the mixer. (There's also a monitor trim control on the same Monitor tab that I mentioned before.) If you're driving a stereo amplifier, you have two outputs, so you don't need a splitter, which you would have needed on the Mackie. If you want to daisy chain active PA speakers from a single output channel, that's fine too.
Directly above the Monitor Out knob, you'll find a a switch that will allow you to switch the output to the Monitor Out jacks from the mixer output to the input from the 2TR In RCA input jacks just above it. This makes it easy to wire in an external sound source like a CD or MP3 player to the mixer without using valuable input channels. When you want to play canned music between acts, just press the switch and start your player to route the output to the house.
If you need to mix the external sound from your CD/MP3 player with input sound -- say, for instance, someone is singing along with a pre-recorded track -- there's a switch just to the left of the Monitor/2TR In switch that will allow you to route the input from the 2TR In to channels 15 and 16 of the mixer, so that you can incorporate it in the mix normally. Just don't be planning to use those two channels for anything else just then. :)
And now it's just a matter of making sure that you've got the buttons pushed when you need them.