We'll create fresh WordPress site with Simple Taxonomy WYSIWYG installed. You have 20 minutes to test the plugin after that site we'll be deleted.
This is a very simple plugin that will convert the taxonomy/category description textarea to a WYSIWYG (TinyMCE) form.
Once active, it will convert the textarea field in the create new and edit page of a taxonomy/category to a WYSIWYG or standard WP content form.