i tried everythin, but i don't have a gw address and dns address. what i know is that is a local network. (192.168.0.3 < this is the ap address).
but how more??? (gw, dns)
You need to observe the data being sent over the network to see who is sending traffic to what. Observing some DHCP traffic would be a great start, but then, if you could observe some, you likely could use it as well.
You can use any dns servers you want (most of the time).
As for the gateway, I would try 192.168.0.1. If that doesn't go, (and you still can't get your sniffer going) then you can either contact the owner of the ap (which you *should* do) or you can just keep running through addresses. Doesn't take that long.
download any one of the freeware ping sweep utilities, run it against the network to get what ip's are available. Unless the user shuts down their dsl/cable modem, you should be able to quickly isolate which is the gateway.