Your Airport Extreme doesn't have anything to do with a wireless mouse/keyboard.
If it's an Apple wireless keyboard and mouse, those are Bluetooth.
What model/ghz is your iMac? Bluetooth was optional on the first generation G5 iMac's, but it looks like it was standard on the following generations.
Go to your System Preferences panel and look for Bluetooth. If it's listed, you have it. You still need to turn it on and pair the mouse and keyboard with the iMac to get them to work. The iMac won't identify them without you putting it in discovery mode. You can do that by clicking on the Bluetooth symbol on the System Preferences page and then click the little + on the lower left to add a device. Your iMac should start searching for a Bluetooth device to connect to. Make sure your mouse or keyboard is turned on and follow the on screen instructions to pair them with the iMac.
If your wireless mouse and keyboard are just regular wireless (infrared or rhf), they should have come with a USB dongle, which you can plug into the iMac and they'll work. But I suspect that since you are posting here you have a Bluetooth combo.
Apple did make a Bluetooth card for that iMac, but we need the model number/ghz to figure out which card it would take. I'm not certain if a simple Bluetooth USB dongle will work on your iMac. Those can be found dirt cheap, so it might be worth trying ($2-3 on eBay).