Parts per million. The R820T dongles tend to drift off frequency and need to be corrected by using the frequency correction function in SDR#. Click on the gear icon on the SDR header. A screen will pop up "SDR controller". At the bottom of that screen, you'll see a small window with a label next to it that says "Frequency Correction". Adjust that up or down until you are on the correct frequency.
For the connection to DSD+, you have to make sure that the audio output of SDR# matches the input of DSD+. The way I found it easiest to find this is to just run the DSDPlus command from the DSD window. it will show you the audio input and output options. Then you type the one that matches in using the -i switch when you start up DSDPlus.
This video does a good job explaining how to connect DSDPlus to SDR#.
https://youtu.be/A2tpDhnuRHY
There is a lot of trial and error, so don't get frustrated.
I also found out today, that the FMP part of DSDPlus does a much better job of processing signal from the dongles than SDR# does.