In order to add new transactions you have to fill out at least the 'Day', 'Origin Account' and 'Target Account' fields, the 'Add' button will then enable itself. Typical error is letting the 'Origin Account' or 'Target Account' field empty (or equals). As iCash performs direct account to account single transactions rather than double-entry ones it will not let you add a new transaction until you submit valid origin and target accounts.
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