mesh? syntax question.

Woe...

 var go : gameObject;

So that's problem 1 that I'm having. Problem 2 is figuring out how exactly to use MeshFilter.Mesh. Again, I'm borrowing code from a unityAnswers topic that gives me this:

 go.GetComponent(MeshFilter).mesh = Instantiate(Resources.Load("Models/Turrets/machinegun"));

But when I run the function that the above line is found in (that is, the function that "upgrades" my object), I get a runtime error saying

"Object reference not set to an instance of an object

I'm hoping this means that I'm just not using the syntax properly. I have the mesh I'm trying to use in the Assets folder under Assets/Models/Turrets. Am I using the path syntax correctly, or is that wrong somehow?

I'm also open to other solutions, though I do need the mesh to be changed from within the script of its own object. Thanks for your time!