Step 1: Check linuxwireless.org.
bcm4318, according to http://linuxwireless.org/en/users/Drivers/b43 , is supported by the b43 driver.
Step 2: Check if your driver is already installed.
That driver is included with the normal Ubuntu kernel image, and is probably already installed on your system.
$ dpkg -S b43.ko
If it were in a different package, you could search http://packages.ubuntu.com for the package it's in, then USB-stick that package to your desktop, then double-click on it to install.
Since the kernel module is likely already installed, what seems to be the problem? Not recognized? Not working? Working poorly?