To install a 3rd party set of Photoshop brushes, double-click on the .abr file with Photoshop already open. The brush set will append the currently open brushes and become available for subsequent sessions. Alternately, you can click on the Menu icon in the upper right corner of the Brushes palette and choose Import Brushes. Navagate to the abr file and click it to append to the list of available brushes.

In recent versions, the classic default brushes that many of us have come to love have disappeared. Do not dispair. They have not been removed - they are simply not installed by default. Go to the same Menu icon and click on Legacy Brushes to be prompted to restore them to your install. *Presto* Many brushes will be added, and you might do well to delete a few - and perhaps entire categories - to pare down on choices.