As I understand it, I need to set the "shift" setting under the
"Radio" tab in SDRSharp to -120 MHz, which is -120,000,000.
Is this the only setting that I need to change in SDRSharp?
I'm not familiar with SDR# in particular, but it makes sense that that would be the only setting you need to change.
If you don't change that setting, it doesn't mean you won't see any signals — it means that (if the upconverter is functioning properly) the signals on the air will appear at frequencies 120 MHz higher than otherwise, from perspective of the RTL device and therefore your software.
One check if your upconverter is functioning would be to tune to 0 MHz (with shift set) or 120 MHz (without shift set) and see if you see a big spike on the spectrum — that should be the upconverter's local oscillator.
(Did you plug the upconverter into USB power?)
Does this mean I need to disconnect the upconverter and disable the shift when listening to frequencies in the normal range of the RTL-SDR?
Yes. Even if powered off, the upconverter contains filters which are appropriate for upconversion and which will prevent normal reception.
My own solution to this problem is to buy a second RTL-SDR device.
Further troubleshooting notes:
Remember that the only thing the shift is going to do is change the displayed frequencies. You can always leave it set at 0 and try tuning around 120 to 180 MHz to find the upconverted spectrum.
Have you set up an antenna suitable for HF? Even a piece of wire strung across the room haphazardly will probably do better than a stock RTL-SDR antenna for this purpose.
Remember that HF is often a noisy environment, and you may have devices producing local noise. It's possible that everything is fine but you're only seeing the noise. Unplug the antenna — if the signal level drops all across the spectrum, then you know you are receiving successfully, you just are seeing the noise that's there. If nothing changes when you unplug the antenna, then you have an inadequate antenna, a nonfunctional upconverter, or you haven't managed to tune/shift to the right part of the spectrum.