You might be better off trying to install the b43-fwcutter application from Synaptic first. The link that you are looking at contains the most recent source code for various wireless drivers. However the link that you are wanting is located in the "Directly downloading the tarball" section about less than halfway down on the page. If you click on it, it should download to your Downloads folder.
Originally Posted by peeterparker
If you do want to install b43-fwcutter from the Terminal:
sudo apt-get update
sudo apt-get install b43-fwcutter