For starters, you’re loading in a jQuery plugin (Fancybox) before you’ve actually loaded jQuery itself, so you’re getting a bunch of console errors from that.
Make sure you look at the source here: http://masonry.desandro.com/demos/infinite-scroll.html
And also remember that audio and some video posts won’t work out of the box and you’ll need to provide a fix for them in Infinite Scroll’s callback.
There are a few Stackoverflow questions about this that should already have answers. Here’s one that I gave: http://stackoverflow.com/questions/12404807/tumblr-audio-video-infinite-scroll/12481862#12481862