I've just installed your plugin - I'm running WP 3.5.2 and Genesis 1.9.2.
When I go to the plugin page (ie. the "404 Page" within the Genesis menu), it looks like this:
As you can see, it's not extending horizontally to the width of the browser window - there's a ton of white space on the right, and the TinyMCE toolbar is chopped off.
In comparison, when I'm editing a Page or Post, the TinyMCE editor extends to the full width of my browser - nice and wide. So I'm not sure why it isn't doing this with your plugin.
Anything you can do to fix this?