Uninstall iTunes, and download it again from Apple. Then install the one you downloaded. You will not lose any music, apps, etc. I had the same problem. It is caused by a corrupt file or the mobiledevice file not working.
To add to my post above, navigate to
/Applications. Delete MobileStore.app and reboot. For safety, make a copy of it and put it in /var/mobile/media/downloads. I'll try it right now and tell you if it works.