You'll find the option to turn it on under Users in the WordPress dashboard. When you get there, click on the Your Profile tab and you'll find it.
But I have to tell you that it doesn't work very well, and you may still not see the option, or see only some of the options. This is apparently a known bug with WordPress 2.1, so I wouldn't be surprised to find a 2.1.1 out eventually.
And as the other person mentioned, the rich editor doesn't function properly in Apple's Safari browser or others using the same rendering engine (such as OmniWeb).
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