Is this new unit or trying to connect to a existing unit?
If it's already been programmed then BOOTP may be disabled.
If new make sure the computer and plc are directly connected, its best to do without a router or switch unless you know for sure there is no DHCP server running. Make sure you disable antivirus, run BOOTP tool with administrative privilege. You should see the MAC address in list show up.
If you do not know the IP you'll have to use a packet sniffer like Wireshark to possibly see the broadcast IP when the controller boots.