There is no official WordPress plugin yet that lets you add the plus one button to a WordPress site, but there is a way to do it. You just have to edit the code snippet from Google a little before you insert it on your WordPress site.
How to add plus one button to a WordPress site
But if you want it on the blog-page, the archive pages the tag pages and all the other pages on your WordPress site, you have to modify it a little before you insert it. But it is easy, change the code to this before you insert it:
<g:plusone size=”tall” href=”<?php the_permalink(); ?>”></g:plusone>
Put that snippet in single.php, in page.php, in index.php, in category.php, in archive.php and in all the places you want it. Be sure to put it in the WordPress loop (just above <?php the_content(); ?> for example).
You can as well change the word “tall” to “medium” or “small” if you want another design of the button.
That easy it is. Good luck and remember to “plus-one” the posts you like!