HOW TO connect wireless NCI to AP via CLI

Hi, i use madwifi as the wireless NCI driver, and it works fine in UBUNTU,
but now for some reason i wanna know how to connect to a specific AP via
wireless tools such as "iwconfig, iwlist" in CLI.

i type "iwconfig" then i see the infomations below.

lo no wireless extensions.
eth0 no wireless extensions.
wifi0 no wireless extensions.
ath0 IEEE 802.11g Nickname:""
Mode:Managed Frequency:2.412 GHz Access Point: Not-Associated 
Bit Rate:1 Mb/s Tx-Power:18 dBm Sensitivity=1/1 
Retryff RTS thrff Fragment thrff
Encryption keyff
Power Managementff
Link Quality=0/70 Signal level=-96 dBm Noise level=-96 dBm
Rx invalid nwid:412246 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:0

i do this follow these steps:

step1:ifconfig ath0 up
step2:iwconfig ath0 essid "wireless"
step3:dhclient ath0

i got the following trace:

then i got the following trace and fail to connect to the ap.
==
There is already a pid file /var/run/dhclient.pid with pid 7734
killed old client process, removed PID file
Internet Systems Consortium DHCP Client V3.1.3
Copyright 2004-2009 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
wifi0: unknown hardware address type 801
wifi0: unknown hardware address type 801
Listening on LPF/ath0/d8:5d:4c:06:f9:9d
Sending on LPF/ath0/d8:5d:4c:06:f9:9d
Sending on Socket/fallback
DHCPDISCOVER on ath0 to 255.255.255.255 port 67 interval 5
DHCPDISCOVER on ath0 to 255.255.255.255 port 67 interval 11
DHCPDISCOVER on ath0 to 255.255.255.255 port 67 interval 18
DHCPDISCOVER on ath0 to 255.255.255.255 port 67 interval 7
DHCPDISCOVER on ath0 to 255.255.255.255 port 67 interval 19
DHCPDISCOVER on ath0 to 255.255.255.255 port 67 interval 1
No DHCPOFFERS received.
No working leases in persistent database - sleeping.
==

i have no idea what happens......  so could someone help me? thanks a lot.
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