By default, the NAS looks to get an IP address dynamically from a DHCP server which in this case is probably your router. Assuming this is a new box, otherwise you might want to do a factory reset should it have been used on someone elses network.
If it doesn't show up anywhere, the only thing I can think of is to start pinging the first addresses in your DHCP pool. Those details would be in your router config. The router might even show the device.
Whatever happens, you need to be on the same subnet as the NAS.