I know there are probably a few step by step instruction already but i would like to share the one i use and hopefully it helps someone.

-First install ubuntu in to window or just run the live cd (google ubuntu) and then run ubuntu os
-plug in your ps3 controller and turn it on by pressing ps button
-open fire fox and type in

http://tomasz.sterna.tv/2010/02/play...is-controller/

or if your on this page in ubutun click on link lol

what you do next is copy and paste
tip (copy short cut is ctrl+c and past in terminal is ctrl+shift+v)

into terminal and hit enter (note you have to be super user/admin) it will prompt for admin sudo password

after this is instal

find your blue tooth mac address in you n900 by going to setting the device information

copy and paste the following command and replace the mac address with yours

after this ps3 controller default primary device should be your n900.

unplug ps3 controller

go on to your n900 open browser and go to the same website stated above (or come to this site and click on the link above)

go down the the green arrow and click on it and it should install the necessary files needed for your n900.

i recommend turning off your n900 and turning it back on before precedding next step

after you turn you n900 back on what you want to do is turn on the blue tooth then hit ps3 ps controller. look at your n900 the top left corner your blue tooh icon should turn blue. now go into any emulator and go to setting and try changing your button by pressing on your ps3 controller (note: the light on your ps3 controller shold stop blinking once the icon turns blue.

here is a video of same text explaination (coming soon typing on this from my n900 and the qik program not working as intended)