The polish sculpt tool is in the course files.
https://cgcookie.com/course/fundamentals-of-digital-sculpting/#files
You can append it to your existing blend file. (shift F1) > locate the file you downloaded and unzipped. (lesson_00_start.blend) > left click it > left click (Brush) > left click (Polish) > left click (append from library). You should have it now.
There is a course on how to customize your start up file to include your custom tools.
https://cgcookie.com/tutorial/customizing-your-startup-file-in-blender/
