Hi eliot.
Basically, you connect the SPL meter to the "line-in" of your PC's sound card, then the sound card's "line-out" to your receiver's analog input. REW will generate a sweep of tones to the receiver that can then be measured by the microphone (the SPL meter) that's connected to your PC.
You'll have to connect the sound card out to the receiver with a stereo cable. Similarly, you'll have to have a stereo cable going into the "line-in" port of the sound card. You will only connect to one side of that cable with the SPL meter as it's a mono output. REW defaults to the left, so it's probably just easiest to go in on that side.
As you may know, the top of every HTS page is a link to the REW manual. There's so much good information there! If you haven't had a chance to check it out, have a read and see what you think. It looks like this, and you can click here as well:
rewlogo.png