Thanks for the question, and welcome to Apple Support Communities.
With iTunes 11 the "Radio" feature has relocated from the sidebar to a sub-section under the "Music" menu:
Tune in to Internet radio stations
1. Choose Music from the Library pop-up menu.
2. Click Radio.
3. To see the available stations, click the triangle next to the type of music you want to listen to.
4. To tune in, double-click a station.