yes im rooted, but the method, i dont understand...
I must mount and, then copy files from zip inside? like nitdroid? and then what i have to do with the kernel?
Let's get this start and over with.
(Better remove your existing NITDroid installation AND multiboot first, you can ask me for detils if you get stuck)
First download all files except the audio fix (there's 4 individual files in the "esentials" link and you need them all)
Now you have 2 deb files for uboot, a 273MB .tar file for CM9, a "uImage" and a "50-cm900.item".
Open 50-cm900.item with any ord editor you like, change the "ITEM_USEATAG" to "ITEM_OMAPATAG" and save.
Connect to PC and put all files under root folder of storage (/home/user/MyDocs)
Disconnect from PC.
Fire up Xterm, type as below and (of course) press enter after each command:
root
cd /home/user/MyDocs
dpkg -i u-boot-tools_2012.04-1_armel.deb
dpkg -i u-boot-flasher_2012.04-1_armel.deb // Install uboot-tools first for dependency
cp 50-cm900.item /etc/bootmenu.d
cd /
mkdir /and // Assuming you haven't installed NITDroid before or have removed it.
mount /home /and
tar xvf /home/user/MyDocs/(name of tar file).tar // Extract main NITDroid system like you've done in N12, etc.
u-boot-update-bootmenu
After these steps just POWER OFF (use the key, not any software switches). After a while slide out keyboard and turn on. Boot menu will appear. Choose the right item and you'll boot up.
Enjoy

(If this's still not clear enough just post your question here, Sanjeev and fellow devs can answer probs I can't)