I am trying to accomplish the same thing. It allows your graphics to be automatically swapped with the retina display friendly version.
You need to upload the retina.js script to the website (I assume to the JS folder) and then paste the line of code<script type="text/javascript" src="/scripts/retina.js"></script>
to the bottom of each page.
I don't know where to paste the code so that it gets applied to all pages. Anyone done this?
I am currently using it on the Afterburner 2 theme.