It may be your username. For most ISP's I've used you need to add "@netzero.net" or "@netzero.com" or whatever.
For instance my login name at Hypercon is:
<myusername>@hypercon.net
One of the reasons I chose Hypercon over Netzero is that Hypercon tells you how to log on, although it may not be easy to find. Maybe if you look around long enough on Netzero's site, you can find it.
By the way, it seems all dialup ISPs are bad, except maybe EarthLink, and EarthLink costs $22 per month. EarthLink once had an offer of $10 per month for six months, so I signed up. It worked pretty well but after the price went up to $22, I cancelled. Maybe if I strike it rich, I'll sign up again.