I tried to install the ACPI from http://acpi.sourceforge.net/download.html, but some of the commands on the download page didn't work for me.

The first part of the instructions, about verifying the signtaures, seemed to work, with no error messages.
For the "patch the kernel" section, I tried to cd to the directory it suggested, but MEPIS said I don't have that, so I put it somewhere else--the only subdirectories /usr/src had were /modules and /rpm . So I cd'd to modules, because I really had no idea where else to put it. (Nope, I don't know what I'm doing here. I just want my machine to be able to hibernate, so I was aping the instructions on this page.)

Then I entered the bunzip command from the instructions. I didn't comprehend any of the output, but there didn't seem to be any error messages here, either.

I got hung up on "make menuconfig." It said only: "make: *** No rule to make target `menuconfig'. Stop."

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Can anyone tell from this what I did wrong?

acpi is already part of the mepis kernel

Really? Well, that's good, but in that case, how do I make the machine hibernate? (Or enter low-power mode, if that's the best it can do.) I've looked in the power section of the control panel, and if I recall, all I saw were laptop power management controls.

hibernate doesnt really exist in linux yet, the best we have is suspend2ram which will require you to compile a kernel with the suspend2 patch.