There is no need for a plugin to add the +1 button on your site. All you need to do is:

Place this tag in your head or just before your close body tag:

<script type="text/javascript" src="https://apis.google.com/js/plusone.js"></script>

Place this tag where you want the +1 button to render:

<g:plusone></g:plusone>

For further information visit the Google official page