You have to get C# scripts to compile earlier so Javascript knows about them. See the docs: http://unity3d.com/support/documentation/ScriptReference/index.Script_compilation_28Advanced29.html

Eric5h5's comment is important about compilation order, if you're trying to disable a c# script from a Javascript script.

However it possibly sounds as though you're trying to re-write this in C#. If this is the case, the problem is happening because "GetComponent" returns a type of "Component" (which Javascript automatically casts to the correct type for you, but c# does not). In C#, you need to do either this:

SampleCode code = (SampleCode) GetComponent(typeof(SampleCode));

or use the newer generic version:

SampleCode code = GetComponent<SampleCode>();

Then you'll be able to use:

code.enabled = false;