Insert the following into your header
<link rel="stylesheet" href="/styles/dhtmlgoodies_calendar.css" media="screen" />
<script language="JavaScript" type="text/javascript" src="/javascript/dhtmlgoodies_calendar.js"></script>

If you didn't install the software in your root web you will need to update the paths in the above statements

Place the next two lines were you want the check in and check out fields to display

<!--{html_xml_cal end_year="+1" prefix="Date1" time="$checkin"}-->

<!--{html_xml_cal end_year="+1" prefix="Date2" time="$checkout"}-->