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’
Click on the blue button '+ New Expense'
This will open a new page, click on the tab 'Mileage Claim'