This method shows promise, but after a couple of reboots the device can't locate networks.
I've been trying for 3 days now to get my wifi to work on MacAir 2008 1, 1 of which has a broadcom wifi device BCM4321 PCI_14e4_4328. Absolutely none of the advice garnered from researching worked for me. But doing the following which I pieced together from different sources worked.
sudo apt-get remove --purge bcmwl-kernel-source
sudo apt-get install firmware-b43-installer
Then the next part is not particularly user-friendly but easy enough:
Download a backport from here. If you select last modified to get the latest on top choose that. Download the build and copy it to a USB stick. Then copy it to your home directory
~/Documents doesn't matter then
sudu su to be root, enter the directory you unzipped and do a make install, it will show and error but it gives you a clue what to do. Can't remember now, but I think it said
make oldConfig it's one of the two notices you get at the beginning.
Here is the link to some instructions to build the backports and install.
Getting the backports installed is the key to successfully configuring your wifi device.