I think that what happens is that the router is not provided the clients with an IP address via DHCP.
The idea is that if you don't have an IP address, you cannot connect to the router, that is a problem!
Wireless routers usually give IP address in the 192.168.0.x or 192.168.1.x ranges so you can try to assign an IP address yourself to your laptop/PC and if you get it right (you have to guess at this point or simply ask the person who owns the router), it's going to work.
Look at the link I've provided if you don't know how to do this stuff. You'll have to enter an IP address instead of using DHCP. One more thing, you'll have to provided your own DNS servers (the one that takes names like yahoo.com and convert them to IP addresses like 10.10.1.2)