If you are using E-junkie Cart buttons, you would paste our version of the GA tracking code on every page where you want to use GA (it's not compatible with the standard code from Google, so don't mix'n'match them). Do not use any GA plugin for WordPress; in fact, the Ultimate GA plugin is known to conflict with our cart buttons. This code needs to be pasted into the raw HTML code of each page somehow, and only appear once on each page; you may need to edit a common template file, such as a footer, to place the code there.
If you are using our Buy Now buttons instead of Cart buttons, then you cannot use our version of the GA tracking code, nor our cart's built-in support for Ecommerce Tracking. In this case, you could use any WP plugin for GA, and you would need to add the standard GA tracking code to your thank-you pages.
Either way, for conversion tracking you should bear in mind the issues covered here: