Next, you have to figure out how to get your "Stereo Mix" / "What You Hear" set up in Windows. Some audio drivers prohibit this. For those, you will need a "Virtual Audio Cable" program, that will create a loopback audio device for you.
In PDW, cycle through modes until you are in FLEX mode, then open Options and select all FLEX options, I believe in the latest version they're checked by default.
In PDW, go to the Interface menu, and go to Setup. For your Input device, select your "Stereo Mix" or "Virtual Audio Cable" device.
Open SDR#, in your Front End, select the RTL device and hit Play. Change your Center frequency to 900,000,000 and start scrolling upwards in the frequency until you can find some strong signals. These are in narrowband FM (NFM).
Once you tune into a good signal, if PDW recognizes it, it should decode the pages like in this video.