Wow, nice, you got this far!
Let me check the source code, then I can tell you with which .net version it has been compiled. OK. The thing is, I truly believe that if you have installed the latest .Net Framework you do not need an earlier version. Well, the source code says (line 313: https://github.com/Memorix101/OpenC1/blob/master/OpenC1_MonoGame/OpenC1_MonoGame...) what you need is 3.5 SP1 (https://dotnet.microsoft.com/en-us/download/dotnet-framework/net35-sp1). So, if this fixes the issue, I have to correct the dependency list on the home page.