A: 

Your namespace "myapp" cannot directly contain the function "ToTest", there is a Module defined there. Try

Imports myapp.LinqExtensions

and make sure it is a Public Module

Willem
I can't import the module (an error is showing up in the IDE). I think this is because modules implicitly import their names into the current namespace.
did you define the module as a Public Module? If not it would not be accessible from another assembly.
Willem
You're absolutely right. If look closely enough I see that "public" is indeed missing... now it's working. Thank you very much!