Getting Theme element into plugins page editor in wp admin

I’m having a hardtime integrating a theme editing element (in default page/post editor in wp-admin) into page/post editor made by the plugin BU Versions.

Basicly I need this theme element – https://www.dropbox.com/s/6z675qfklvwk8vd/theme%20element.PNG
to get in the plugin editor –
https://www.dropbox.com/s/v1z0ltobraj3b6l/plugin%20editor.PNG

I’ve been googling and the only thing I found that could be relevant was this plugin wiki.

I don’t really know what to do with that since I know only HTML, CSS and some Javascript atm, and don’t know where to look.
Any help much appreciated.

Luke