Well, it depends on the version of AS you're running, but the old getURL() will always work. Check the help files for it's use.
In first frame of a layer, referencing a button on the stage named "myButton":
_root.myButton.onRelease = function(){
getURL("some-file.html");
};
on the button itself,
_this.onRelease = function(){
getURL("some-file.html");
};