Note:

This applies specifically to Photoshop CS3 and CS4 but may also apply to other versions of Photoshop as well.

• Open Photoshop
• Click on the Brush Tool
• At the top, Click on the small arrow next to the currently shown Brush
• At the top right of this menu, click on the small arrow that faces to the right
• To add a brush(es) to your current selection of brushes, Click on Load Brush and the select the brush you would like to add (*.abr)
• To replace all of the brushes and have the brush(es) you select only, Click on Replace Brushes (You will be prompted to save before replacing) and then browse and select the brush(es) you would like to add (*.abr).