Taken from
http://blog.roberthallam.org/2008/04/broadcom-4318-ubuntu-hardy-heron-ndiswrapper/
1) Remove the b43-fwcutter package
sudo aptitude remove b43-fwcutter
(NOTE: If you do not have b43-fwcutter enabled in the Restricted
Drivers Manager, ignore this step.)
2) Install ndiswrapper
sudo apt-get install ndiswrapper-utils-1.9
(NOTE: If ndiswrapper is already installed, ignore this step.)
3) Download and install wireless driver. Extract the contents to your
user's home directory.
Wifi drivers available at
http://www.megaupload.com/?d=LMXHHJQ9
4) Create bash script to fix wireless
cd /etc/init.d
sudo gedit wirelessfix.sh
(NOTE: If using Kubuntu, use kwrite in place of gedit. If using
Xubuntu, use mousepad in place of gedit.)
Paste the following into the empty file:
#!/bin/bash
modprobe -r b44
modprobe -r b43
modprobe -r b43legacy
modprobe -r ssb
modprobe -r ndiswrapper
modprobe ndiswrapper
modprobe b44
5) Save it, and close the text editor.
6) Make wirelessfix.sh executable:
sudo chmod 755 wirelessfix.sh
7) Execute the following command:
sudo update-rc.d wirelessfix.sh defaults
8) Go back to your user's home directory.
cd /home/user (substitute user with your username)
9) Go to where the drivers contents were extracted. It should be a
directory named wlan.
cd wlan
10) Install the drivers.
sudo ndiswrapper -i bcmwl5.inf
sudo ndiswrapper -i bcmwl5a.inf
11) Reboot.
Wifi should be working upon reboot. Enjoy!