The problem is you're trying to share one IP address among multiple computers.
Your DSL modem supplies a connection to your ISP, but does not establish an IP address. By adding the hub to the DSL modem, you are allowing multiple CPUs to access the modem, but your ISP only gives you one IP.
So net is, you can't get IP through Airport, especially since Airport doesn't support PPPoE (from the card - it will through a base station).
What you need is a router that can establish a PPPoE connection through your DSL modem and share it to your network. Linksys has one that will do this.
Have the router establish the PPPoE connection, then distribute IP addresses via DHCP through your wired network and your wireless access point.
Network looks like:
DSL modem connected to line out
Router connected to modem and establish PPPoE connection (has 4 ports)
Wireless AP attached to router.
CPUs happy because all get IP addresses.