<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/09/13/checking-to-see-if-a-flex-checkbox-is-selected-before-allowing-a-user-to-press-a-button/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.controls.Alert;

            private function button_rollOver(evt:MouseEvent):void {
                if (!checkBox.selected) {
                    button.errorString = "You must agree to the EULA before continuing.";
                }
            }

            private function button_rollOut(evt:MouseEvent):void {
                button.errorString = "";
            }

            private function button_click(evt:MouseEvent):void {
                if (button.errorString.length == 0) {
                    Alert.show("Continuing...");
                }
            }
        ]]>
    </mx:Script>

    <mx:CheckBox id="checkBox"
            label="I have read and agree with the EULA..." />

    <mx:Button id="button"
            label="Continue"
            rollOver="button_rollOver(event);"
            rollOut="button_rollOut(event);"
            click="button_click(event);" />

</mx:Application>