I found the
online help index at hometheatershack.com very helpful.
In my setup, I ran a SPL meter (sometimes a microphone) to my sound card (coincidentally, the exact SoundBlaster Live external card shown in the REW help manual), then I ran the output to the 990. To answer the question on the level, the REW software has soundcard and SPL meter calibration wizards that will help you set the right volume levels. In my case, the levels were much more tame than the 990's autocalibration (plus you have full volume control).
Although I was able to send the test tone to the 990 via the USB port when using the 990 as a sound card, I was not able to crack the code on getting my laptop's onboard sound card to leave its microphone port enabled at the same time and gave up on that experiment. It would have been neat as heck if I could have used the 990's microphone input to feed the REW. I was never satisfied with the performance of the sound cards at my disposal and, as I mentioned earlier, my impatience got the best of me and I opted for the SMS-1. Hopefully you'll have better luck.