WYSIWYG editor missing from Scheduled Reminders popup window in Civi 4.6.3
Answering the question of how to install the TinyMCE editor extension:  It's marked as compatible with CiviCRM 4.7+.  I'm guessing it relies on functionality that doesn't exist in the current version of CiviCRM - but you could always e-mail Tyrell directly and ask him.

Assuming it works - it's submitted to the CiviCRM extensions directory, but hasn't been approved yet.  So first, make sure you have your extensions directory and URL defined (one is in Administer menu > System Settings > Directories, the other in Administer menu > System Settings > Resource URLs).  Then place the extension in a folder inside the extensions directory (use FTP or git or whatever works for you).  At this point, you should be able to go to Administer menu > System Settings > Manage Extensions and select "Install" for the extension.  If it works on 4.6, report back here please!