I just downloaded the file and installed. I am not seeing the add on anywhere, not even in options. I am pretty sure I am running OneNote 2016.
OneNote DM tools for Dungeons and Dragons 5e · By
That looks like the right version. So if it's not showing in the add-ins section in the options, some people have had issues with the user-level registery entry not being made.
https://www.dropbox.com/s/y30klazl11usz79/AddIns.zip?dl=0
Download this file, unzip it, and right click each file and pick "merge". You might need to restart your PC after this. See if that doesn't solve it.