Note that for this step to work correctly, your plug computer needs to have access to Internet. Maybe you can just SSH into it, and ping www.amahi.org to make sure it works.
Find the plug computer IP address (check your router DHCP logs or DHCP server), and launch the Amahi installer by visiting http://plug_computer_ip:2000
(Use nmap or ping -b to find new devices on your LAN, if you can't find your plug computer IP address on your DHCP server.)