Err, okay. It doesn't matter much at all what the function does, the way you call it will be the same regardless.
To make a function visible to external code, you have to assign it as a property of its container object. So you'd do this in the root timeline:
this.myFunction = function(){ this.mc.gotoAndStop(1); }
Then call it from your external code like this:
exportRoot.myFunction();
Of course, if that's literally all this function will ever do, you could just skip the function and do it directly from your external code:
exportRoot.mc.gotoAndStop(1);
Although there are advantages to maintaining an abstraction layer between internal and external code.