I am using almost the same setup as Ciscokid. (I mean the RS digital SPL meter and SoundBlaster Live! 24-bit USB, but not same AVR, I have a Marantz SR5500.) I went through the same steps with the same results. My interpretation of what's happening is different from the interpretation of Brucek. I think that the error message you are getting at the "Set Input Volume" step is unimportant, and you should go on to the next steps and take measurements.
Here is what I mean:
(1) When you "Set Measurement Level" (the first step, after connecting all the cables and turning everything on) you set the volume control on the AVR to get a reading of 75 dB on the 80 dB scale on the SPL meter. During this step, you either leave the Master Volume Control on the SoundBlaster card at 50% of full scale, where the program automatically sets it, or manually raise the SoundBlaster volume control to 100% in the Surround Mixer panel, as suggested in this thread.
Note that 75 dB on the SPL meter is also
"-20 dB relative to full scale" in REW, in other words 95 dB on the SPL meter would be equivalent to "full scale" or "reference level" or "0 dB relative level" in REW. I found this concept confusing at first, but I from what I've read, I think this is standard practice for audio and recording engineers (I mean, defining a "full scale" or "reference level" and then giving SPL levels in dB relative to that reference level).
(2) Then the next step (before any other calibrations or measurements) is Set Input Volume. In this step, for some reason that I can't understand
scratchhead: ), REW tries to raise the volume of the pink noise tone to get a reading of -18 dB relative
(or 77 dB absolute) from the SPL meter, rather than -20 dB relative where you just set it
in the step before.
But maybe REW cannot
raise the volume from -20 dB to -18 dB relative, either because the SoundBlaster volume control is already at 100% and can't go higher, or because, even if the SoundBlaster volume control is less than 100%, REW cannot adjust this control ("some soundcards do not provide programmatic access to their input volume control").
So you get an error message which is effectively saying "I'm unhappy because I couldn't automatically adjust the volume control to raise the volume from -20 dB to -18 dB, but that doesn't really matter much, so ignore my complaint and keep going".
(Let me know if my interpretation is wrong, I just did 30 separate measurements, 5 speakers * 6 positions of SPL in room, and I think the results are good in spite of always getting the "Set Input Volume" error message.)