I'm doing some testing on 4.6 and after upgrading a site, I don't have the option to click into the HTML section when creating a mailing. The Plain Text section seems fine.
I have cleared caches and even tried changing the WYSIWYG editor to "Text Area" to see if it was something related to the editor, but it's the same:
Does anyone have an idea on how to get the CKEditor to show?