To add a transaction all you have to do is click on the button below your current balance section. A form will show up and you will need to input the information asked there (i.e: type of transaction, date, description and amount)
Here's what the button looks like:
To edit a transaction you need to go to the month where you created said transaction and click on it. It will prompt a form where you'll be able to edit any field you had previously filled.
To see your monthly balance, hover your mouse over the chart on the month you wish to check. It will show you the entire month's balance.
To select a month, just click on any of the buttons containing the shortened month names.
Here's what the buttons look like: