The "acpi=force" is a kernel parameter, which "forces" loading the acpi modules (you'll get a better explanation from somebody else, I admit, but in short that's what it does -- like "acpi=off" ought to disable acpi). That needs to be passed to the kernel when it is booted, and thus it is added to the bootloader configuration - Lilo configuration in your case (some systems use Grub which is, in my opinion, a lot easier to configure). Not sure why it is not loaded by default, but anyway..acpi deals with power management (like apm, which is more or less deprecated I guess), and if it is not loaded, your system can't automatically power off - it will just halt, and will power off when you press the button yourself. That's why you need to load it.
The lilo configuration file is usually /etc/lilo.conf and it's a plaintext file (i.e. not encoded) which you can edit - as root, or with root privieges - with any text editor of your choice. The syntax is rather easy, so that should not be a problem. After you have edited the file with a text editor of your choice, added the acpi=force option to the kernel parameters list among the other ones and saved the file, you need to run
as root, which "installs" lilo to MBR, saving the configuration. Without running it no changes take effect. After doing that reboot and see if it works. If you used Grub as your bootloader, you would only need to modify /boot/grub/menu.lst (same sort of thing, the file syntax just is slightly different, though not difficult either) and that would be sufficient because Grub reads it's configuration from the disk.