Call a function in another class

Hey I just threw myself into the as3 fire this past monday on a project at work, not only that but I decided to be cool and use classes, which I have only a small amount of experience with, anyways, I couldn’t really find an answer to my question so hopefully some kind soul could help me out.

basically, I want to call a function that is in an external class:

so I have my main class which is good, but then I want to call a function in another class, in as2 you’d do it like this foldername.foldername.asfile.functionname, but it doesn’t seem to want to work in as3.

any thoughts?