Finally got "Check for Updates" to respond by switching, back and forth, which USB mouse I used on my PC running Windows XP.
Thanks to other posters, I learned how to use the View menu to make the sidebar appear as in previous iTunes. But when I clicked Apps and saw the 2 buttons in the lower right corner ("Check for Updates" and "Get More Apps"), the "Get More Apps" button responded, but the "Check for Updates" button was unresponsive, as reported by other Windows users elsewhere in this forum and on other websites (Attention Apple Support: this is a REAL problem encountered by multiple people using iTunes 11 on Windows XP Service Pack 2!! Please fix.)
I tried shutting down and restarting iTunes, with no success. I did not try rebooting my laptop or uninstalling and reinstallilng iTunes. I noticed a discussion in the MAC (not Windows) area of the forum, where several people solved the unresponsive "Check for Updates" problem by switching their mouse. I turned off the power switch at bottom of my wireless mouse (without removing the USB receiver from the laptop) and connected a wired USB mouse, but the "Check for Updates" button was still unresponsive. I disconnected the wired USB mouse, turned the power back on for my wirelss mouse, and VOILA, the "Check for Updates" button is now working! I cannot explain how my whacky workaround worked, but some of you might have the same success if you remove your mouse (or turn off its power switch if a wireless mouse), attach a hardwired mouse, then remove that mouse, and reconnect your original mouse (or turn back on its power supply if it's a wireless mouse.)