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:
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.