I'd suggest going back to ISP and get them to clarify the situation. For your situation, both machines need an external (internet) IP address. I'd suspect your ISP has only provided one - and that is tied to the Mac (did you have to go through a set up process to get the mac connected?). So, when the PC tries to connect, there's no IP address for it to pick up - hence the message.
So, if they are really providing 2 addresses, you will probably need to go through a process of assigning the second IP address to the PC.
PS. Its more usual to have a router (rather than hub)connected to broadband (single IP address) which then shares connection to machines on LAN, acting as DHCP server (& firewall).