You must not have something configured correctly then. If you run our .NET Reflection QuickStart, use Add Reference to add System.Core, then add "using System.Linq", then add a string declaration in a method, and do "myString." you'll see all the Enumerable extension methods.
Make sure you follow all the "key steps for getting started" in the main .NET add-on help topic exactly as they are specified. That should get you working.