Where's the Custom Field area (swiffy plugin help please)

Hi. So someone has made me an animated graphic. It has been converted from Flash animation into html5 code. So now i have a huge chunk of code.
I’ve downloaded and installed swiffy plugin. It’s telling me i need to add a custom field. But that’s not an option showing up in the new page i’m trying this on.

Where is it? / How do i get the *Custom Field* box to show up so that i can complete stage two of the plugin please

Thank you