See https://rdotnet.codeplex.com/workitem/95:
"However, it works with VS 2013 but does not work with Xamarin Studio 4.2.3, the code is exactly the same. Since Xamarin Studio 4.2.3 only supports x86, I change the path to C:\Program Files\R\R-3.0.3\bin\i386\R.dll but it does not work. The error is the same."
Clarify: not running in Xamarin Studio (debub mode; hosted process?). Targetting which CLR: .NET or Mono? etc.