Easy. Using a Wired computer, go to http://192.168.1.1/ and log in using the username of admin, and the password being the router's serial number (CASE SENSITIVE). Go to Wireless Settings, and from here under Basic Settings you'll find the Wireless On/Off Switch. If Wireless is on, make sure SSID Broadcast is enabled. You will find that under Wireless Security.
Worst case scenario, if you don't have many custom settings to lose, just factory reset your router. In almost all cases, within a minute your router will be back to the state in which Verizon gave it to you when you first got FiOS.
