I have noticed a 'Stickey' post on the Steam forums that does talk about how this DLC should be accessed. Hope a link to their forums is not out of place here.
In my case, none of that happened. Flight never showed up in my game library so I could not right click on it get to the keys. I never had any notification of unredeemed codes after logging into Live, nor on the map or in the hangar. It seems Steam just never assigned those codes to me after purchase, which is why I have initiated the Paypal dispute.
Rather than "Have any of you tried Steam", my original question should have been "Have any of you successfully purchased DLC through Steam for Flight that was initially installed outside of Steam". I have not seen anyone say they have successfully managed it, but I have seen one other say they had tried, but nothing happened. Just like it didn't happen for me.
BTW, the Maule is quite a bit of fun in gusty conditions and a crosswind landings if you first remove the check marks from 'Increased Stability' and 'Propeller Effects'. It no longer feels like it is on rails. Way more fun! And spend a little time over Pearl Harbor, or on it with the Icon. Quite a bit of detail there. You can actually see the Arizona below the surface.