Hi Elliott,
I've updated the package today to include a read me containing instructions on how to overcome this issue.
It seems I forgot the include an important piece of information required for the Async plugin to function, the run-time version of C# for your project must be set to 4.0 or above. This can be done by going to Edit -> Project Settings -> Player) and changing “Scripting Runtime Version” to “Experimental (.NET 4.6 Equivalent)" or "4.X"
If you continue to run into issues, please hesitate to get in touch. If I miss your comment for whatever reason, you can reach me at ctinney94@gmail.com