Martina et al.,
I've had a similar problem, and Martina's point was right on for my situation. Only admins were denied the editor.
I tested her theory by creating a new user to see if the editor worked for this user. It did. So I went back and deleted all users and had them create new accounts. Fortunately, this is a new site with only a few users. It worked. Now, even as an admin, I have the editor.
I have no idea why this worked, but I'm glad it did. The Xinha solution is a good one but beyond most end-users in my experience.
This was a site originally installed by a friend of mine using Fantastico, which installed Moodle 1.5.2. I upgraded the site to 1.53 and added some extra modules and themes.
Hope this helps.