You cannot change a British Airways to an Air Canada using ACE.

To add an aircraft, you use either the ACE, by using Add New. Or you can add it manually.

The instructions for using ACE to add a livery is explained in ‘767 Captain’ FLIGHT MANUAL Part I – User’s Manual (cs767_manual1.pdf) starting on page 38. If you don't have the manuals, you can download them from here: http://www.captainsim.com/products/b767/manuals.html.

If you want to, or have to, add it manually and don't know how, just ask and I'll provide instructions on how to do it.

Mark