App Inventor consists of several different parts that work together. These parts live in various places: on Google's servers, on your computer, and on your phone. To install the parts that run on your computer, download and run App Inventor Setup. This document tells you how to do that.
In most cases, App Inventor should be able to locate the installed software on its own. But if it asks you to type in the location to look for it, the path name to enter is /Applications/Appinventor/commands-for-Appinventor
Something not working right? Visit the troubleshooting page, or check the App Inventor User Forum for help.