If you are wondering where your iDisk went after the move to iCloud, go read
Apple's support articleBut what if you do not see your iDisk in Finder ? Well that's easy, open Finder and press ⌘K and enter 'https://idisk.me.com/yourMobileMeName' as the Server Address. Without the quotes of course and yourMobileMeName should not include @me.com, just your name. Click the Connect button and your iDisk will open, it might ask for your credentials first.