1. Click on the payments link on the navigation bar near the top of the page, or, in the index.
2. This will take you to a page detailing your expense and payment history. (This screen will also appear as the last step in any of the items that you are purchasing (e.g. buying parking permits or renting lockers.)
3. Scroll down to the bottom of the page and you will see an option to make a payment online.