I was stumped by this. One of the new fea­tures of iTunes 9 is Genius Mixes:

Meet Genius Mixes. This new fea­ture searches your iTunes library, finds songs that go great together, and cre­ates mul­ti­ple mixes you’ll love. All auto­mat­i­cally. These mixes are like chan­nels pro­grammed entirely with your music. You may dis­cover songs you never knew you had — and redis­cover for­got­ten favorites.

But after upgrad­ing to iTunes 9, I didn’t see the Genius Mixes in my playlist bar. I found the (now obvi­ous) answer over on MacOSXHints.com:

If you update to iTunes 9 and don’t see the Genius Mixes (even though you have the Genius enabled), sim­ply select Store » Update Genius.

Once the Genius update is com­plete, the Genius Mixes entry should show in the sidebar.

I’m com­forted to note that even Rob Grif­fiths — THE Mac expert behind OSXHints.com — was stumped by this one as well.