Installing the Windows software for App Inventor Setup has two parts:
NOTE: App Inventor 2 does not work with Internet Explorer. For Windows users, we recommend using either Chrome or Firefox as your browser for use with App Inventor.
You must perform the installation from an account that has administrator privileges. Installing via a non-administrator account is currently not supported.
If you have installed a previous version of the App Inventor 2 setup tools, you will need to uninstall them before installing the latest version. Follow the instructions at How to Update the App Inventor Setup Software.
In most cases, App Inventor should be able to locate the Setup software on its own. But if it asks for the location of the software, the path to enter is C:\Program Files\Appinventor\commands-for-Appinventor. If you are using a 64-bit machine ((How to tell if you're on 64-bit Windows), you should type Program Files (x86) rather than Program Files. Also, if you did not install the software as an administrator, it was installed in your local directory rather than in C:\Program Files. You'll need to search for it to find the correct pathname.
Something not working right? Visit the troubleshooting page, or check the App Inventor User Forum for help.