Basically, in order to restore your backup taken with backupmenu, the kernel first needs to be the same one that was in use when you took the backup. So, from a phone you've never laid hands on before, you

• make sure your backup is somewhere else, like on a microSD
• flash to PR1.3 (you shouldn't have to flash eMMC, but, if your backup is elsewhere, you can...)
• install backupmenu and latest kernel-power first
• insert microSD, boot to backupmenu, restore from backup
• reboot into maemo

I wouldn't try flashing just a Nokia kernel if you were using kernel-power previously. Flash PR1.3, then allow kernel-power to do its thing, then restore your backup.