embedded projects can be used on other websites and blogs like this

http://scratch.mit.edu/projects/embed/13621660/“ frameborder=”0" allowfullscreen>

almost correct
Why? You see that </iframe> part of the code? It is a closing tag. That means that there must be an opening tag. A part of that opening tag is http://scratch.mit.edu/projects/embed/13621660/“ frameborder=”0" allowfullscreen>, but it's not complete! A few information is missing, for example that we want an iframe, that http://scratch.mit.edu/projects/embed/13621660/ is an URL and we need the source for the iframe and the size of the iframe.

The correct code to embed a project as an iframe on a website is: