Adding an expense is easy: click the red "- Expense" button in the right column from any page, and you'll get the add expense form in a neat popup.
The forum requires the amount for the expense (as well as its currency), the category the expense should be allocated to, the account the expense is paid from and optionally a short note on the expense. Pro accounts can also add tags, attach documents or add a multi-categ expense.
Here are some articles related to adding an expense:
Can I add expenses in other currencies?
What happens if I add an expense to a date prior to a balance update or to the account creation?
Expenses can also be added from the Expenses page by clicking on a day from the calendar.
Editing an expense can be done by clicking the edit button on the expense for any page it would be displayed on. If you can't find your expense in the Dashboard, go to the Expenses page and navigate to the month the expense was made in and you will find it in the list and in the calendar.