Unfortunately, it seems that DBDiff for Oracle is no longer available. Furthermore, the application is so severely outdated that it's a huge security risk for your PC and it won't even work on newer Windows versions.
If you have a newer version of Windows and you actually obtain the installation package, but it refuses to work, apply a compatibility tweak using right click on file then Properties > Compatibility. Select an older version of Windows from the list, preferably Windows XP Service Pack 3, tick the Run as Administrator option as well and then launch it.
In case you need an alternative solution for Oracle database comparison, you could take a look at dbForge Data Compare for Oracle. It has a trial version, so you can check out if it's actually useful before spending any money.