Info for this can be found here
http://www.google.com/+1/button/However it's pretty simple. Any page you want it you can add the following code. In showflat.tpl with the FB and Twitter icons is an obvious start.
In your header file add the following before the </head> tag...
{literal}
<script type="text/javascript" src="http://apis.google.com/js/plusone.js"></script>
<script type="text/javascript">
function plusone_vote( obj ) {
_gaq.push(['_trackEvent','plusone',obj.state]);
}
</script>
{/literal}
The function plusone_vote is a callback if you use Google Analytics, allows you to track events.
Add the following in your showflat.tpl...
<g:plusone callback="plusone_vote"></g:plusone>
