First, please make sure that the backup was created successfully. iBackup Viewer will not show unfinished backups.
Then, you need to check if you have moved backups to other folder than the default location.
Mac: /Users/USERNAME/Library/Application Support/MobileSync/Backup
Windows XP: C:\\Documents and Settings\\USERNAME\\Application Data\\Apple Computer\\MobileSync\\Backup
Windows Vista, Windows 7, Windows 8: c:\\Users\\USERNAME\\AppData\\Roaming\\Apple Computer\\MobileSync\\Backup