About Java 1.5:

• Java 1.5 is not so common-place yet. It is definitely different enough from 1.4 that Impro-Visor will not run on the latter. Also, Mac users beware that the “upgrades” currently being offered by Apple in automatic system upgrades may reset your version back to 1.4, so you may wish to decline such upgrades.

• You can download Java 1.5 free from:
  
  http://www.java.com/download/

What you want is the JRE (Java Runtime Environment) 1.5,

also called “Version 5”, and also called J2SE Runtime Environment 5.0.

For MacOS you should be redirected to something like:

http://www.apple.com/support/downloads/java2se50release1.html

NOTE: Once you install on MacOS, you may still need to run this utility:

/Applications/Utilities/Java/J2SE 5.0/Java Preferences.app

and tell it to use version J2SE 5.0.