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.