I was reading alot of users got problems with or installing,setting up and using the java port of whatsapp.
And i'm here to make life easy again atleast for now.

What do you need and how much time this gonna take
Time 10-30 minutes

phone
Whatsapp java port
Oracle JRE
Filebox
Root
Internet connection

pc
Windows xp.vista or 7
Bluestack
Notepad++

First of all install all the programs that you need for pc and leave them like that.
Then download install root and filebox and also leave them for that now.

I'm assume you already download whatsapp and Oracle JRE.
Rename whatsapp-java-0.0.4alpha.deb to whatsapp.deb and rename oracle-embedded-jre-1.6.0u30.deb to oracle.deb and copy them in the root dir of you're nokia N900.

Now open Terminal X and type the following, pressing enter after each step.
root
cd /home/user/MyDocs
dpkg -i oracle.deb whatsapp.deb

Check if whatsapp icon appears on the phone.
If you see it start the application and fill in the infomation like IMEI etc.
now just press the button below all of that and yes you will get an error that is normal!
Close whatsapp now and launch filebox.
click on the gear looking icon and enable "show hidden files and folders" and press the back button.
now click on the arrow that point upwards and search for the folder ".java"
form there click on every folder until you can't any futher and you will see only one file named "prefs.xml".
Long press and select copy and click on the icon that look as a house (lol) and paste it with the icon that looks like a notepad.

Now connect you're phone to the pc as mass storage and open prefs.xml with notepad++.
delete all lines and copy this
Example: (Thanks to yelram)
imei : 00000000000
CC: 34 (your country code)
Phone number: 123456789

then the file should look like this:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE map SYSTEM "http://java.sun.com/dtd/preferences.dtd">
<map MAP_XML_VERSION="1.0">
<entry key="IMEI" value="00000000000"/>
<entry key="CC" value="34"/>
<entry key="CONTACTS" value=""/>
<entry key="LOGIN" value="34123456789"/>
<entry key="PHONENR" value="123456789"/>
<entry key="REGSTEP" value="1"/>
</map>

and edit the lines that you need to edit and save it.
now disconnect the phone and go filebox and copy the prefs.xml from the mydocs to the original prefs.xml now close all apps and turn the phone OFF!

now we are almost there!
open bluestack and install whatsapp
launch whatsapp and put in the info that you ask for it
now wait 5 minutes and whatsapp will say that they couldt verificate it and that is fine by us
now turn on the phone again and launch whatsapp and on bluestack whatsapp press call me afther the phone fully booted up ofcouse whatsapp will call you and you will get the code
fill the code in on the phone wait for couple of mins and bam whatsapp will work

this is the most easy way i can give you enjoy whatsapp port on you're N900!
(will clean the tut later and add some pics also)