Got it figured out. I'm using v2.08 if it matters.
1. Start RivaTuner and open up the hardware monitoring window, shown upper middle of pic.
2. At the bottom of that window hit
Setup, it will open a Hardware Monitoring Setup window shown upper right of pic.
3. In the list of data sources, highlight
Core temperature and hit
Setup right below that. That will bring up another setup window shown at the bottom left of pic.
4. Near the top of that window you will see a drop down menu beside an icon labelled
Data Provider. The box beside it should say
default. Open the drop down menu and select
NVThermalDiode.dll:0 and hit
OK
The thermal monitoring should now be working.