If you choose specific devices, outputs and inputs from the REW selection boxes you have the option to always adjust volume settings yourself via your soundcard's mixer or to allow REW to set them to the last used values on startup. This is determined by the check boxes below the output and input selectors. Note that REW may not be able to control levels under some operating systems (e.g. Mac OS X) or for some soundcards, in those cases leave the Output Device and Input Device set to Default Device.
Make sure that the Input Channel is set to the channel you are using to connect the SPL meter (or mic preamp output).
It is a good idea to measure the soundcard using the button in the Calibration section of the Soundcard Settings before making measurements to allow the response of the soundcard itself to be compensated for.
Before making measurements use Check Levels to make sure the replay and input levels are set correctly.