Update Genius enables the Genius Mixes feature
If you don’t see Genius Mixes in iTunes’ Library list choose Store > Update Genius. iTunes will gather new Genius information for your Library from the iTunes Store, and automatically add the Genius Mixes option to the Library list for you.
Genius Mixes shows your 12 pre-made mixes
Once iTunes finishes building your mixes, just select Genius Mixes from the Library list and click on the mix you want to play. Rolling your mouse pointer over a mix tells you what type of music it holds, so it’s easy to get an idea of what you are about to listen to.
Genius Mixes is a great new addition to iTunes, and will likely come in handy at plenty of parties — or for writing all night.