It is often useful to log in to a customer's control panel, for example to assist with configuration or replicate an issue that the customer is reporting.
You can do this by logging in to your administrator control panel and selecting the Billing link in the top right corner....
...and then selecting Customers from the left menu.
A list of your customers will be shown, alongside the one you wish to login as simply click the icon on the far right.
Alternatively, from within the customer account itself select To Control Panel on the General tab.
If the customer is reporting an issue with a particular administrator's account, you can login in as that particular user. From within the customer's account, select the Users tab, and then the administrator....
...then click the To Control Panel link.