For new tenancies, you should never need to add amounts due because the system will automatically take care of all this in the background.
However, when adding an existing tenancy which started before you were using Tenancy Manager, it might be useful to also add their rentals.
You can manually add a rent due by viewing the tenancy record and at the bottom to the right of the rent schedule is a "add rent due" button.
On clicking, you enter the rent due date and the amount due. On save, this will be inserted into the rent schedule.