If you just started to use the app here is the instruction how to write down income.
You'll need to tap the wallet icon (top right corner), choose the wallet or expense where you'd like to add income and insert the sum and the date of income ("Today", "Yesterday" or "Other date").
If you are a premium-user and you've turned on multiply incomes mode, you'll have an arrow instead of icon in the top right corner. Tap on this arrow and you'll see income sources line. You are able to add as much income sources as you need to have. To add new income you need to drag down the coin into the wallet as if you were writing down outcomes.