1. To add a transaction on the iPhone app, tap "+" in the bottom left.
2. Enter the "Amount" and "Merchant". The date will be filled in automatically, but can be changed by touching the "Date" field. You can also choose a Budget Item by touching "Choose Budget Item".
3. Tap "Save" in the top right. The transaction will be saved to the Budget Item that was selected. If no Budget Item was selected, a coin will appear on the main budget page.
You'll notice the number on the coin increased from 4 to 5 after adding the transaction. That number shows how many untracked transactions there are. Touch that coin and you will see those transactions. See this article for details on assigning those untracked transactions.