use xarchiver to unrar (or just unrar from a PC and copy them into your n900).

after extraction, you should see a .desktop file and 4 images. there is one more image called preview. you can ignore the preview image.

copy all the 4 images and the .desktop file into /home/user/Images.

then go to your desktop, tap the top to edit the desktop, in the menu, select "change background" and you should see the image set appear in the list of backgrounds.