One or more types required to compile a dynamic expression cannot be found

Solution -Add Reference -> .NET -> add Microsoft.CSharp.

Two things are important. Check the following:

  1. That your project properties are referencing .NET Framework 4 or higher (if that is not the case, change it: Right click on the project, select the "Properties..." menu item, then select as target framework ".NET Framework 4" (or higher))
  2. That you have added the "Microsoft.CSharp" assembly as reference (note that step 1., which requires to re-load the project, must be done before you can do that)

Afterwards you can use the dynamic type. The reference needs to be added in the selected project or in the startup project in order to become available in the immediate window's scope.