first install nitdroid auto-installer and when its installed from menu run nitrdoid un install. when it done removing previous nitrdroid then from menu run install nitdroid and it will do the rest. you can get auto-installer from here

if you dont want installer to download N12_UMay.tar.bz2 and nitdroid kernal rc6 then proceed to this link and download these two files and put them in n900. not in any folder or they will not get detected. now auto-installer will do the rest remember nitdroid will be installed in eMMC. for more information and help proceed to this thread: Click Here for Nitdroid thread.

Happy Installing.