Hi, sorry for the delayed answer, i'm just back from vacations.
First of all, create a script file that will hold your callback functions, then, create the callback function itself, inside of it.
(Basically, the script files are treated like containers for callback functions, you don't call the file itself, but the functions inside of it).
For example:
function MyCallbackFunction() {
/// That's the body of your function
}
Then, when you add elements to your menu, just pass the name of the function, since it's inside a script, you can call it from anywhere, the name will be orange (a way to make sure you type it right).
Example:
var _main = Add_Group("Main Menu");
_main.Add_element(GGL_sub_button, "DoSomething", MyCallbackFunction, _dx, 0, _w, _h);