If you have DHCP running on the network, you should just be able to plug it in and it'll obtain an address. If it's not working from this, you can check it's address by plugging a phone into the unit and pressing: ****
It'll say something like 'Configuration menu' or some crap then to check the IP, press: 110#
Enable/disable DHCP with 101#
See if DHCP is enabled with 100#
Set an IP with 111#
Set netmask with 121#
Set gateway with 131#
Once it has a valid IP either way, just hang the phone up and the box should do it's work and you should have a dialtone or something. Keep in mind it needs about five minutes to fetch it's settings, etc from Vonage.
Other than that, I'd RTFM or call Vonage.
Good luck!