1/1/09
Note:
This applies specifically to Photoshop CS3 and CS4 but may also apply to other versions of Photoshop as well.
Click on the Shape On the Toolbar
Select Custom Shape Tool

Custom Shape Tool
Click on the Arrow next to the shape in the top menu
The Shapes pallette will drop down.

Shapes palette
Click on the arrow on the right and select load Shapes

Load Shapes
Browse and select the *.csh (Custom Shape File) you want to use.
The shape will be available in the Shapes Palette when you click on the arror next to the Chstom Shape Tool