From Showit Desktop Manual

Showit is an Adobe Air application that relies on installation of Adobe Air to run. If you don't have Adobe Air, it will automatically download and install as part of the installation process. Go to the Download and Install page and click on the "Install Now" button. This will take you through the install wizard to get Showit running on your computer.

If you have Installation Issues, check out this page for information on how to resolve it.