To show or hide the iTunes Sidebar, click the "Show or Hide the iTunes Sidebar" button in the lower-right corner of the iTunes window:
Using Genius and Ping with the iTunes Sidebar
The iTunes Sidebar can display information from both Genius or Ping (both of which need to be turned on first). Information from both Genius and Ping will display depending on available data from the store. If there aren't any Genius recommendations for the song you selected, then Genius won't be displayed; the same is true for Ping.
For more information, see the following links:
You must have an iTunes Store account to turn on Genius.