How to change class variable from inside a movie clip

programming flash
1

I declare a Boolean variable in my main class (Main.as file)

public var myboolean:Boolean;

What I want is to change myboolean variable from inside a movie clip I’ve got on stage (when it reaches a certain frame).

I write

myboolean = true;

but what I get is “Access of undefined property”

How should I write this?

(the variable is accessible from Stage)