Hello simoneta10,
iTunes 11 looks a little different than the previous versions of iTunes, and I'm wondering if you do not see the Genius Mixes because the Sidebar is hidden. You can follow these steps to bring back the sidebar in iTunes:
Show Sidebar - Choose View > Show Sidebar (this feature will display iTunes features on the left side of the application including Library, Store, Shared, Genius, and Playlists).
You can find the full article here:
iTunes 11: Frequently used features
http://support.apple.com/kb/HT5649
Additionally, I recommend reviewing the following resource to sync the Genuis Mixes to your iPod:
iPhone and iPod: Understanding Genius Mixes
http://support.apple.com/kb/HT3843
Thank you for using Apple Support Communities.
Best,
Sheila M.