The AirPort Extreme 802.11n Enabler requires AirPort Extreme Update 2007-001 or later. If you attempt to install the AirPort Extreme 802.11n Enabler without first installing the AirPort Extreme Update 2007-001 or later, this alert will appear:
"You cannot install AirPort Extreme 802.11n Enabler on this volume. This volume does not meet the requirements for this update."
Download and install the AirPort Extreme Update.