If you just want the button and nothing else, you can use the following bits of code as is:
The first tag goes into the header.php file, inside the theme found in the packages folder. The second tag goes into the a page.php file, the header file, or the footer depending on where you want it to show up.
That's all there is to it. Unless....
You don't necessarily want all those +1s going to random pages on your site when they could help your landing page or home page. Also, you might want a larger or smaller button or to get rid of the count. If so, head to: http://www.google.com/webmasters/+1/button/index.html and you can configure the button as needed to update the tags.
That's the quick and dirty way to add them. If someone has a better way to add across a site or make it easier to get the appropriate layout, feel free to build off of this how-to.