Can’t load a class into WordPress post editor’s content field

You can’t, not the way you are trying. You can’t start meddling in the iframes internals, it has its own DOM etc that needs to be respected etc

This question is already veering into offtopicness as it is more of a TinyMCE question than a WordPress question

Instead create a tinyMCE plugin, include jQuery in that plugin, and then use standard jQuery to add the classes you need. The tinyMCE plugin js will be inserted inside the editor dom head tags inside the iframe.

At this point it becomes a standard JS jQuery problem, and moves beyond the scope of this website.

Here’s a tutorial on how to create TinyMCE plugins ( just be sure to indent the code correctly:

http://ctx2002.wordpress.com/2008/07/28/how-to-write-a-tinymce-plugin-for-wordpress/