After you installed a new Linux Mint Petra Release, change the sources.list (/etc/apt/sources.list) like the following suggestion and you’ll be able to install for example the Broadcom WLAN driver.
First comment out all the old entries with #
then add the following lines and save it.
deb http://packages.linuxmint.com/ petra main upstream import deb http://archive.ubuntu.com/ubuntu/ saucy main restricted universe multiverse deb http://archive.ubuntu.com/ubuntu/ saucy-updates main restricted universe multiverse deb http://security.ubuntu.com/ubuntu/ saucy-security main restricted universe multiverse deb http://archive.canonical.com/ubuntu/ saucy partner # deb http://archive.getdeb.net/ubuntu saucy-getdeb apps games
sudo apt-get update && sudo apt-get upgrade -y
on your command line.