Where can I setup iCloud in USB Disk?

First make sure you are signed in iCloud: open the Settings app and go to the iCloud section. If you are signed in then a "Synced files" or "iCloud" section will appear on the home screen of USB Disk. Files and folders you place in this section will be visible in USB Disk from all your devices (note that USB Disk will not let you access files other apps place in iCloud).