@peter i think your instructions should start like this :
if you dont have java on your pc go here and install java
http://www.java.com/en/download/index.jsp
next right click the whatsapp file and open with java
proceed with the entering of details

on device:
enable extras testing repository (if not enabled) and look for icedtea6 (or higher)
install
place the same whatsapp jar file on the n900 (documents)
(then the rest of your instructions)