<?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 isChecked():void {
                if (!checkBox.selected) {
                    Alert.show("You must agree to the EULA before continuing.");
                } else {
                    Alert.show("Continuing...");
                }
            }
        ]]>
    </mx:Script>

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

    <mx:Button label="Continue"
            click="isChecked();" />

</mx:Application>