Installation from nuget requires manually adding the following to Package.appxmanifest. Ideally the nuget package would take care of adding and removing this block. This extension information outlines all the classes exposed by libsodium-uwp.dll. Additionally the output of the build should construct this information based upon Sodium.h.
Presently if this data is not added to Package.appxmanifest builds will error out with a class not found error.