These instructions are for Adobe Photoshop and Photoshop Elements.
Go to Edit > Preset Manager. A Preset Manager Dialog Box will open
Click LOAD and point your computer to the ABR file for the brushes you wish to use. Click OK.
Your Preset Manager will now include thumbnails of the brushes you just loaded. Click DONE.
Create a new layer in your document. (Layer > New > Layer)
Select the Brush Tool (B). In the options toolbar, click the down arrow next to the current brush thumbnail.
In the drop down menu, you can select your brush and set its size (master diameter). You can also adjust the size quickly by using the [ and ] keys.
Once your brush is selected, click once to "stamp" the image on your blank layer. Click and drag to "paint" with your brush using your foreground color.
Be sure to play with the settings in your PS Brushes Palette (Window > Brushes), or in your PSE "More Options" drop-down menu on the options toolbar. You can adjust the angle, size, spacing, and so much more.
Brushes can also be used with many of the painting and retouching tools, such as the Eraser, Pencil, and Clone tools.