Open AirPort Utility
Hold down the option key on your Mac while you double-click on the AirPort Extreme icon
Wireless clients will be displayed. Click on the small arrow next to each client for more information about each device.
Unfortunately, Ethernet clients are not displayed
In order to set up Reserved IP addresses for your devices, the AirPort Extreme must be the device that is handing out IP addresses. To verify if that is the case.....
Open AirPort Utility
Hold down the option key while you double-click on the AirPort Extreme
Click the Network tab at the top of the window
The setting for Router Mode must be at DHCP and NAT in order for the AirPort Extreme to assign IP addresses.
If you see a Router Mode of Off (Bridge Mode).....then another device on the network........likely the device that you call your "modem" is acting as a router for the network. If that is the case, then you will need to set up IP address assignments on that device.......not on the AirPort Extreme.