Adjust the AV processor volume (preferred) or the Sweep Level (if connected directly to an equaliser, subwoofer or amplifier) to obtain the desired SPL reading on your personal SPL meter. Generally this will be 75db. Next, adjust the Input Volume either using the REW Input Volume control or your soundcard's mixer or OS audio level controls so the input level is between -12 and -24dB FS, ideally around -18dB FS. If you are using a mic preamp, the level control on it may need adjusting. Note that the VU meter is heavily filtered to make it easier to read, it will react slowly to changes in volume setting so allow time for it to settle.
If the input meter reading is very low and does not increase when the calibration signal is playing or the input volume is adjusted check that the correct Input Channel has been selected, that the correct input is selected in the soundcard's mixer or OS audio controls and that the cabling is correctly connected.