It can be difficult to understand how to install brushes, so I will explain here.

First download the brushes you want, you can get some good ones from

http://browse.deviantart.com/resources/app...ions/psbrushes/

After you download the brushes, in Photoshop go to Edit>Preset Manager.
Then make sure it is set to brushes, click "Load" on the side.

Locate where you downloaded them to, for Firefox default is Desktop.

CLick the .abr file and it will load. Click done, and you're ready to use your new brushes!