I see you mention it is a dynamic IP account. If so, the first time on you need to authenticate using the software the provider gave you.
I helped a friend set up his dynamic IP in his business, the tech he had did not know that the PPPoE software provided by the telco HAD to be run first. He kept harping on the "I need an IP" thing.
First off you must have TCP/IP as a network protocol installed etc. If this is an existing network using Microsoft networking with a server assigning IPs for the network, that is a whole other story. Both my office and home networks use the routers to assign IPs (work is a Netgear RT311, home is Linksys BEFSR411) and I have not had cause to configure sharing where a PC is acting as the DHCP server. So the properties are Obtain IP address automatically, use DHCP, enable DNS with the providers DNSs entered, TCP/IP bound to the NIC.
They must have given you some software, either Enternet 300 or similar. Run that on the PC closest to the dsl modem (except for an NT server. DO NOT RUN ENTERNET 300 ON AN NT SERVER). Take a straight thru patch to the NIC on the pc you want to use, when the software is installed it will "see" the dsl line and connect to a secure site. You will enter something like regdsluser as username and reguser as password, then it will prompt for you to enter your own username and password. (read the instructions that came with your install kit).
After all that is done, then plug your Linksys into the dsl modem and the network hub, fire up a browser, go to 198.162.1.1 (the Linksys interface), enter the Linksys password (admin is default) and set it up to get dynamic IP from provider using PPPoE (username and password entered once here) and everything should be right! Then uninstall the PPPoE software they gave you. Very important if you are going to use a router to log in.
Sorry if this is all steps you may have already taken, don't overanalyse too much. If all else fails put TCP/IP back to default settings etc and start from square one, following the providers instructions to the letter, and take not what step fails and let me know. My friend's big problem turned out to be that while the dsl line was good, it had not been provisioned correctly at the d-slam! So it would not talk to the PPPoE software at all.
Madkiwi