Ethernet has a self-assigned IP address and won't work

I run Snow Leopard on a late 2009 Mac Mini and so far have put up with poor wireless reception in a cottage with thick stone walls. But today I bought a pair of homeplugs and eagerly plugged them in, awaiting a stronger signal through the LAN sockets. But it doesn't work coming up under Networks with the message "Ethernet has a self-assigned IP address and will not be able to connect to the Internet." How to I go about reconfiguring it?