The channel will face down/inside, the more flat side faces up/out. When you drive the old one out with a hammer and a long screwdriver from below (the whole reason the valve body needs to come out), you will see how it's installed. Just drive the new one in from the top. BTW, how much did the stealership charge you? I seem to remember I got one from a tranny shop for next to nothing.
Mike
Mike