Since a few of the last WordPress upgrades the Visual editor toolbar you get with wordpress broke and its taken me a while to find a solution but heres one that works.
Use your FTP Client yo login to your hosting account
Delete the following two folders;
- wp-includes
- wp-admin
Re-upload them from the same WordPress version you have installed (i.e the 2 folders you deleted)
Empty your browsers cache, and retart your browser.
This should fix it.
Source;
http://wordpress.org/support/topic/wp-25-wysiwig-problem-tinymce-does-not-start?replies=18