For older versions of Alice, we have received reports from people in which they are receiving a "Java not found" error message when they try to install and / or run Alice 3.1, and they have Java installed on their machine. If this happens to you, it will be very helpful if you perform the following steps:
You should know that the latest versions of Alice no longer require a Java installation.
See the following link:
http://bharatsoft.blogspot.com/2010/08/setting-path-and-classpath-in-javahow.html
There is a difference between the Java JDK, which you need for Alice, and the Java JRE. Make sure that you have installed the JDK.