The Custom Text function only works on the front end (what the user sees); it does not appear to follow through to the back end. For instance, I have changed "Postcode" to "Zip Code". Appears fine for the user, but if I go in the back end to add a booking through the admin interface, the form still shows "Postcode".
For the control panel you can exchange any text in SUB ACCOUNTS (top right). Click on 'Manage Account' for the account in which you want to exchange test. The setting to exchange text is 'Control panel custom text'. For instructions how to us this function please use the context help (? icon) next to the setting.