Okay. For those who have power kernel installed and want to be able multi-boot between power kernel and Nitdroid:

0. You need a blank MicroSD card in the N900.

1. Install as per steps mentioned in this thread. (both for nitdroid and power kernel) This will install 0.0.8 and will setup multiboot with 3 options. (original, power kernel, Nitdroid) No need to reboot after both steps.

2. Download and put the 0.0.9 tar.bz2 file from first post in this thread and copy it to /home/user/MyDocs/ (root of eMMC internal 27GB partition)

3. Run following from xterminal while being root:

mount /dev/mmcblk1p2 /and
tar xvf /home/user/MyDocs/NITDroid-0.0.9.tar.bz2 -C /and

4. Done. Now reboot with slide open to see the multiboot menu.