This very simple script displays a message box showing some text.

<?xml version="1.0" encoding="UTF-8"?>
<scriptfile>
    <settings program="actiona" version="3.8.0" scriptVersion="1.1.0" os="Windows"/>
    <actions>
        <action name="ActionMessageBox" version="1.0.0"/>
    </actions>
    <parameters/>
    <resources/>
    <script pauseBefore="0" pauseAfter="0">
        <action name="ActionMessageBox">
            <exception id="1" action="0" line=""/>
            <exception id="0" action="0" line=""/>
            <exception id="2" action="1" line=""/>
            <parameter name="windowIcon">
                <subParameter name="value" code="0"></subParameter>
            </parameter>
            <parameter name="customIcon">
                <subParameter name="value" code="0"></subParameter>
            </parameter>
            <parameter name="type">
                <subParameter name="value" code="0">ok</subParameter>
            </parameter>
            <parameter name="ifNo">
                <subParameter name="action" code="0">do_nothing</subParameter>
                <subParameter name="line" code="0"></subParameter>
            </parameter>
            <parameter name="message">
                <subParameter name="value" code="0">Hello world!</subParameter>
            </parameter>
            <parameter name="textMode">
                <subParameter name="value" code="0">automatic</subParameter>
            </parameter>
            <parameter name="title">
                <subParameter name="value" code="0"></subParameter>
            </parameter>
            <parameter name="ifYes">
                <subParameter name="action" code="0">do_nothing</subParameter>
                <subParameter name="line" code="0"></subParameter>
            </parameter>
            <parameter name="icon">
                <subParameter name="value" code="0">none</subParameter>
            </parameter>
        </action>
    </script>
</scriptfile>