For the confused (I initially only saw a sign-up form and thought it was required, and came back here disappointed) - if your browser window is below 1300px or so wide, the live demo won't show up. When it does show up it is quite subtle - in the screen of the generic background image is an editable code snippet, transformed with CSS so that it is skewed and appears to be in the picture. You can type and run code in that laptop window screen.

Note to the site author - you should put the live screen flat below the sign-up form even when the screen is smaller. Changing window size should not remove information from your page, just move it around.