At first you need to download the MadWiFi driver. You can get it from sourceforge.net. You have to download it via CVS with the command:

cvs -z3 -dserver:[email protected]:/cvsroot/madwifi co madwifi

Then you have to compile the driver by changing to the folder madwifi. Just type "make all" and then "make install". Now the drivers are automatically loaded during boot-up.
Temporarily you can set an IP by "ifconfig ath0 192.168.0.1 netmask 255.255.255.0 up" ( you can choose your own IP).
Then set the gateway by:
route add default gw (router's IP)

Then there should be a wireless inet-connection.