It should be at the top right of deviants' pages, somewhere around the "Give" button. If it doesn't appear, it could be a problem with your version of Chrome or you may not be using Google Chrome (many people try it on Firefox, but so far the extension only works for Google Chrome).
Deviance will be used as a limiter for decks when they're added to the game. You can check out some of the blog posts for information covering a lot of the planned mechanics for the game in the future (when school grants me enough time to actually do something).