Are you sure the FiOS device is in routed mode or bridged? If it is bridged, then there is nothing you need to do with it. If it is routed, then you will need to forward a port through the Verizon device and then through your router before it will work.
I have read that the defaults on those are admin and password, but that could very well be wrong. In order to try to connect, look at your router's WAN settings. If you have a public IP, you are in bridged mode and you won't be able to get into it, but there will also be no need to port forward. If your router has a private WAN IP, then it's default gateway will be the address you put in you browser to connect to the Verizon device.