You need to connect openSPOT to your local area network, which has a DHCP server (home routers usually have DHCP servers enabled). After that, you can use
http://openspot.local
to open the web interface. If it still won't open, you can take a look at the DHCP clients list on the router's web interface to find out openSPOT's IP address (or you can use scanner software).
If you plug openSPOT to your PC's Ethernet port, you need to start a DHCP server on your computer as by default, openSPOT operates in DHCP client mode.
