I ran into the same problem. I went and downloaded
akpi from here:
akpi and download the rpm for Mandrake 9.1.
I went to the Boot section of the Mandrake Control Center and Clicked on DrakBoot and then chose Lilo/Grub mode and clicked on the
configure button. I then put a check mark in
Enable ACPI and clicked
OK.
I then went to System --> DrakServices and ensure that acpi and acpid where checked and apmd didn't have a check.
Then I hit the Menu key, clicked on run and entered akpi, voila battery monitor in the system tray.
HTH
<<edit>> sorry about the double post the proper link is in the response following this one<<edit>>