As I learned the hard way after some troubleshooting, there are two important preparations you have to make before using App Inventor.

The first is that your computer must be running Java 1.6 (also called Java SE 6.0). On the Apple website, you can find this update under Java 1.6, Update 4 from July 2009.

Even after you’ve installed it, your computer may still be running an older version of Java. On a Mac, you’ll want to open Applications > Utilities > Java Preferences. Under both “Java Applet Plugin” and “Java Applications”, drag Java SE 6 to the top.

You should be all set, Java-wise.

The second basic requirement for running App Inventor is that you should be on Firefox 3 or greater.