start off by downloading all of the brushes you'd like to install and setting them in a folder somewhere in your home directory or your desktop.
open the terminal, it should be in applications - system tools. start off by switching users to root (su - )
enter your password and type in the following commands so your terminal matches mine:

what you're doing here is first changing your location to the directory with the brushes folder in it. the last line makes the brush folder accessible to all users, letting you edit it as a regular user.
now open up your home directory (it should be USERNAME's home on your desktop). in the bottom right corner you generally should be able to switch to /, the topmost directory. from / go to:
usr - share - gimp - 2.0 - brushes
this is the brushes folder you just edited with chmod. now you can open up the directory you saved the brushes you downloaded earlier and drag and drop them into the folder. next time you open up gimp, they should appear.