Listo relies on iCloud to keep your lists synchronized between all your devices. You just need to log each participating device into the same iCloud account. You set up iCloud in the Settings app on your device. Also ensure iCloud's Documents & Data switch is set in the ON position.

Occasionally, iCloud seems to get confused and lists get out of sync or won't sync at all across your devices. The first thing to try is to make some changes to one of your lists: just cross something off. Updates can take up to 30 seconds to happen.

If updates and lists just don't seem to be appearing on other devices, launch the Settings app and turn off "Documents & Data" for iCloud, then re-launch Listo, then re-enable "Documents & Data" again. That seems to be enough to kickstart iCloud on the device.