Hi, the editor on that panel is the wordpress editor. If you don’t see the editor while composing the newsletter there is for sure a plugin which is creating problem on Newsletter administration panels. It’s common: not well written plugin injects their code on other plugins panel.
I can try to help you in identify such plugin, but I need to access your blog as administrator (or at least as editor but first enable the Newsletter plugin to editor from the main Newsletter configuration).
Nota : you can delete above because i make a mistake in choice of option between Enable access to editors? and Disable visual editors? thus tests was false
Finally, I found that lhe plugin Spider Video Player is in conflict with your plugin.