first of all you must know that ACPI is enabled in your kernel; this isn't a fact always. but you'll see it when you add the line and if it doesn't become enabled, you know you'll have to deal with the kernel first....anyway, let's assume your kernel has enabled ACPI.
usually Grub's "config" file should in my opinion be in your boot partition...and called menu.lst (at least on my pc). so, as root,
and if you don't get any errors telling you that you don't have such dir, do
and see if you have menu.lst or grub.conf or something. if you do, that's our file...now open it with some text editor and locate the line that tells grub about your linux kernel, like
kernel /boot/bzImage root=/dev/discs/disc0/part1 hdd=ide-scsi
or similar (it looks a bit different on your config, of course), and then apply the option at the end of the line like this (using the example line above)
kernel /boot/bzImage root=/dev/discs/disc0/part1 hdd=ide-scsi acpi=on
then save and reboot..grub doesn't need to be re-run/re-installed like lilo after making changes, so that should be enough. and again, if this won't help, then you don't have ACPI support compiled in your kernel..
if you don't have /boot/grub/menu.lst or /boot/grub/grub.conf or like that, you have to search for where your grub is installed..I guess it is usually in /boot/grub, so that's why using some /etc/grub.conf doesn't help I think. except, of course, if you have installed your grub into /etc, but that'd be somehow weird..
so see your /boot/
and one more thing: if you happen to accidentally edit a wrong file, or make a typing error and save it in the real config file, it shouldn't stop your booting...the last option then just (I think) gets ignored, and perhaps you get an error message telling you it's not ok. so you can't mess up your boot with this, unless you really work hard for it