On the left hand menu, click on Projects or Proposals, depending on where you want to add expenses to 

This will bring up a list of all of your Proposals or Projects that you have added, select the one you want to add your expenses

At the bottom, click on the tile that says ‘Expenses’

This will open a new page, you will see a blue button ‘+New Expense’ click this

A form will then open where you can fill in details about the expense such as the amount, date spent on and description

Once you have entered all of the details, click submit expense (if you have Xero linked, this will update your Xero, however it will show as bills to pay rather than expenses)