Try using Spotlight to locate Photo Booth. Hold down the command key and press the space bar; next, type in Photo Booth and see if the Spotlight index is able to locate the application. The default location for this application is the Applications folder. If you do not have the application installed, it is possible that you deleted it by accident. You can restore the application using the installation DVDs that came with your notebook. Insert the first DVD and install software that came with your computer. You should be able to locate the iLife suite and pick and choose what you need from there.
Let me know if you have any questions.