Can you find the rectangle tool? On my PSE3 it's towards the bottom of the toolbar. (I have it on the left side of my screen.) Once you find the rectangle tool, notice the little triangle next to it. That means there are more tools available. Click on the tool and hold. There will be a pop out menu and you'll see several other options. Click on the one you want, in this case the custom shape tool.

__________________
avatar from www.vanilla-dreams.com