As Mike alluded to, the USING statement in your code is only part of the story. Adding the reference adds the DLL to the project, which allows your code to call those functions. By having that reference loaded, the autocomplete will work when you start typing your USING.
The USING statement is a shortener for the namespaces, so if you didn’t want to add the USING you can say GHIElectronics.NETMF.System.MathEx.cos() (or whatever function you want)