To get mine working I had to log in as the administrator on my Moodle site and go to:
Site administration -> Advanced features -> Plugins -> Text editors -> TinyMCE HTML editor -> General Settings.
The editor toolbar field was blank. Even though a Default was specified, it was not working. So, I copied the lines of text from below "Default:" and "Each line contai..." and pasted them into the text field.
I repeated this for the "defaults" under available fonts list.
For me, the exact lines for "Editor toolbar" were:
...and for the "Default fonts"...
Trebuchet=Trebuchet MS,Verdana,Arial,Helvetica,sans-serif;Arial=arial,helvetica,sans-serif;Courier New=courier new,courier,monospace;Georgia=georgia,times new roman,times,serif;Tahoma=tahoma,arial,helvetica,sans-serif;Times New Roman=times new roman,times,serif;Verdana=verdana,arial,helvetica,sans-serif;Impact=impact;Wingdings=wingdings