Someone mentioned futher up this thread if the checkbox under your profile is gone you might want to re-upload wp-includes.
I am just chiming in to say that I just installed wordpress on my webhost last week, and can use the visual editor fine. However another user registered, and I made him an author, however with or without the box checked under his profile his visual editor does not appear in IE7 or Firefox 2.0.0.2.
He runs his own wordpress blog (altho slightly outdated) and his visual editor works there so it is not a browser issue.
