import fl.controls.CheckBox;
import fl.controls.ProgressBar;

var cB:CheckBox = new CheckBox();
cB.label = "indeterminate:";
cB.move(10, 10);
cB.addEventListener(Event.CHANGE, cB_change);
addChild(cB);

var pB:ProgressBar = new ProgressBar();
pB.mode="manual";
pB.indeterminate = cB.selected;
pB.setSize(100, 20);
pB.move(15, 40);
addChild(pB);

function cB_change(evt:Event):void {
    pB.indeterminate = cB.selected;
}