Hi,
Your issue is most likely related to the fact that you are using the HTML 5 version instead of the XHTML version. Your theme is very likely to be coded in XHTML and not in HTML 5.
Add this line of code in the <head> section of your blog in header.php, before the </head> tag:
<script type="text/javascript" src="https://apis.google.com/js/plusone.js">
//<![CDATA[
{ parsetags: 'explicit' }
//]]>
</script>
and place this code where you want the g+ button to appear:
<div id="plusone-div"></div>
<script type="text/javascript">
//<![CDATA[
gapi.plusone.render
(
'plusone-div',
{
"size": "standard",
"count": "true"
}
);
//]]>
</script>
If you wish, you can change the appearance of the button by changing the size parameter and enable/disable the share count. For more information about these paramters, you should check out the g+ documentation.
Cheers!