If you sync with Producteev and you’ve uninstalled and reinstalled (or used the “Clear all data” function), not to worry – you can still get your tasks and sync.
Accounts & Settings -> Premium & Misc. Settings -> Enable Third Party Add-ons
This setting will place “Producteev” back on the Sync page:
Accounts & Settings -> Sync & Backup -> Producteev
Sync between the two services may not be fully compatible at this time, but you can still do some basic syncing using this feature.