Is the AE behind a router/modem? It needs to be the router/modem in order for the "guest" network option to appear.
If not, you can "bridge" your router/modem so that the AE can become the router/modem (essentially). I just found this out myself recently.
If you have the AE with N dual band, enable the 5Ghz band. I recommend enabling that if you have Wireless N compatible devices.
Good luck.