To send lists from Zenkit To Do into Zenkit Base, open the list, click or tap on the list name, go to "Zenkit Suite" and click "Connect" next to Zenkit Base. To send a collection from Base to To Do, open the collection, go to "Zenkit Suite" and tap or click "Connect" next to Zenkit To Do.
If you don't see the "Connect" option or the other Zenkit product in the Suite settings, it means that you haven't yet "installed" that Zenkit product. It's super simple to install it: Log into your Base or To Do account on your browser, then in a new tab, open the other product and click "Install".
For example, if you already have a To Do account, go to base.zenkit.com in your browser to install Zenkit Base, too.
You can learn more here.