The default IP address is 192.168.254.254. You need to set an IP to your computer just as the following steps.
Set a fixed IP to your computer, such as 192.168.254.10. You can follow the guide to configure the fixed IP.
Connect your computer to the LAN port of MoCA adapter via ethernet cable.
Visit http://192.168.254.254, username is admin, password is gocoax.
FYI... the following "how to upgrade firmware page" cites the wrong IP address for the goCoax adapter.
See: How to upgrade the firmware?
Document cites: 192.168.144.200
Should state: 192.168.254.254
(The embedded video correctly states the needed IP address.)