You need to go to your Dashboard and then to Layout and add a gadget - the one you want is Configure HTML/Java Script.
This was the code I used:
<a href='javascript:void((function()%7Bvar%20e=document.createElement('script');e.setAttribute('type','text/javascript');e.setAttribute('charset','UTF-8');e.setAttribute('src','http://assets.pinterest.com/js/pinmarklet.js?r='+Math.random()*99999999);document.body.appendChild(e)%7D)());'><img alt='Pin It' onclick='doPinIt();' src='http://i493.photobucket.com/albums/rr300/blogsbyheather/PinIt.png' style='margin: 0 0 0 0;'/></a>
One last point, if your post doesn't have at least one reasonably sized image, Pinterest won't let me pin it.