Is your Motorola "modem" -ONLY- a modem, or is it a combined modem/router? (also called a "residential gateway")
Look on the back. If it has more than one ethernet connection (there's usually a group of 4), then it's likely a combined modem/router.
chabig has the correct solution in reply 4 above this one.
Use Airport Utility to change the setup so that the Airport functions "in bridge mode".
It works VERY well that way.
I've been using an old (flat) Airport Extreme with a Uverse residential gateway for 7 years this way, with nary a hiccup...