Extension Methods not Recognized

Referencing an assembly containing a class with extension methods is not enough. You need to import the namespace containing the class in each of your source file where you want to use the extension methods.

For example, to use LINQ-to-objects, you need to reference the System.Core assembly and import the System.Linq namespace (which contains the Enumerable class with the LINQ extension methods):

using System.Linq;

If the Extension method is callable when not using the Extension syntax, use the Format:

this.MyExtensionMethod()

That cleared up my problem of not finding the Extension method of a class in VS2010.