Download from here the b43 firmware: https://www.dropbox.com/s/n2tjz9jnj9xw5 ... ar.gz?dl=0
(I have uploaded it onto my dropbox account). (If you have ethernet connection do it on your own PC, otherwise you can download it from another PC that has internet connection, and then copy/paste it to your Zorin PC with the use of a USB stick).
As soon as it is downloaded, paste the b43.tar.gz firmware file you downloaded into your home folder
Right click on it and select "extract here". A new file will be created named "b43".
Open terminal (ctrl+alt+T) and write the following commands, the one after the other, pressing <enter> after each one:
sudo apt-get remove --purge bcmwl-kernel-source
sudo mkdir /lib/firmware/b43
sudo cp b43/* /lib/firmware/b43
sudo modprobe -r b43
sudo modprobe b43
Your wireless should start working.