Applies to: Linux Mint 17.3
Because of an apparent bug, if you're using a Broadcom based WiFi card (such as BCM4321) on this edition of Mint, you need an internet connection already established, either from cable all by using an Atheros based USB Wifi dongle. (E.g. TP-722). At least this is so, if you attempt to install from a USB memory stick, made from an ISO using UNetbootin.
What should happen is that you get a screen under driver manager that look like the picture below, then you select the bcmwl-kernel-source and Mint should download and install the new kernel-driver from the USB memory and after reboot it should just work. This does not happen as the driver manager insists on network connection.
So instead find an alternative connection and install using driver manager.

In addition, please note that on some laptops (especially HP) there may be a manual WiFi switch. For some reason, even when on it is starting of as off, so you need to toggle that switch. You can check this status (if the drivers are properly installed) with:
# rfkill list 0: phy0: Wireless LAN Soft blocked: no Hard blocked: no 1: brcmwl-0: Wireless LAN Soft blocked: no Hard blocked: yes
Then, once you've toggled, you should get something like:
# rfkill list 0: phy0: Wireless LAN Soft blocked: no Hard blocked: no 1: brcmwl-0: Wireless LAN Soft blocked: no Hard blocked: no 2: hci0: Bluetooth Soft blocked: no Hard blocked: no
If for some reason, it would show as soft blocked, you can run:
$ sudo rfkill unblock 1 $ sudo /etc/init.d/networking restart
iwconfigand ii) the output oflspci -nn | grep 0280.