When you are logged in, there is a link from the homepage just below the main picutre on the right hand side that says 'book airline awards'. This will search for both AC and *A carriers. Beware, however, that if you're searching between city pairs that AC flies to, even if indirectly, it will sometimes refuse to show you *A options and only show AC ones. Very annoying!

For LHR-LAX it seems to show both, so perhaps they have fixed this glitch...