1. Download the file and unzip it.
2. Copy the .abr file to your Photoshop/Presets/Brushes folder.

3. In your Photoshop Brushes palette, click on the arrow in the upper right and click on “Load Brushes.”

4. Navigate until you find the file and load it (Photoshop/Presets/Brushes/BB_HiRes_Leaves.abr )

Note: The next time you restart Photoshop you will not have to load the file, it will be in the list of Brush Libraries.