You must have ExpenseIn Admin access permission in order to be able to see the account drop down option.
To view your ExpenseIn account number please log in via the Web-Portal:
Click on the Account button in the top right corner of your screen. If you use Linked-Accounts this will be the name of the account you are currently viewing.
Your ExpenseIn account number is displayed in the blue box at the top of the pop-out window.