Let me put in just a couple of notes here:
1. While I appreciate people thinking about using Multiboot and working to make it compatible, Multiboot is somewhat depreciated, and shouldn't be used - It overwrites the kernel partition each time you switch your boot OS!
Remember that the kernel partition is on a raw flash device with -no- block-leveling; if one block goes bad, your whole N900 could die.
uBoot is the replacement, and it simply chainloads a new kernel off whatever partition you want, resulting in -no- extra writes to the kernel partition. It's a far better system and anyone thinking about dual-boot should be using it.
(Note that BootMenu should be compatible with uBoot, though building a BackupMenu module for uBoot(and a boot-menu) would be nice).

2. Manual BackupMenu install would require:
A. Extract "data.tar.gz" from the .deb file(as root). "tar pzvf ./data.tar.gz -C /" IIRC
B. Extract the contents of "data.tar.gz" to '/'. All paths should work out fine.
C. Run these two lines as root:
Everything should run after that.