One option for rich text editor is wysiwyg. To activate it you must install some version of WYSIWYG extension in your mediawiki.
To get toggle link activated (=switch between wikieditor and wysiwyg mode) you must enable it in user preferences (=Use toggle to switch between wikitext and rich editor (replace textarea with rich editor)). To use toggle link, you must also have WikiEditor extension installed.
For more information about different version of wysiwyg see download section. You can find one version of wysiwyg here (version: 1.5.6_0 [B551+12.01.2015]), you can also find short installation and configuration instructions there on front page. This version supports source mode inside wysiwyg editor and also wikieditor extension using toggle link located on top of wysiwyg window. Section Installation also tells something about user permissions.