You need to check the output of the mixer. Most should have RCA, 1/4" and XLR outputs. You will probably need two 1/4" to 1/8"TRS cable. You most likely won't find this anywhere, and will have to resort to multiple cables and adapters, or make the cable yourself with parts from the local Radio Shack. I'm also guessing it'll be live-take recording with no room for error. You'll have to try a few takes before you get it right, as once it's recorded in this fashion, you can't adjust the levels if you find a vocal track stands out too much or the organ is drowned out. Make sure you have a few sets of good headphones as well, because the PA will have to be turned off so it doesn't feed back into the mics. By the way, what software are you using to record?
Jarrett