I have installed slackware on my toshiba not long ago and faced the same problem. I don't know about APM but if you have a bit of time and patience, it is probably better to go for ACPI.
Here is how I did it:
- Get the latest kernel from www.kernel.org
- Take your existing .config file to compile the new kernel. That will prevent you from loosing all your settings if they already work fine. You need to do those 2 steps as by default, slackware doesn't install the kernel with a proper kernel path.
If you don't know how to compile your kernel. see the top thread here.
- download the acpi patch from http://sourceforge.net/projects/acpi
and install it. It will basically patch your existing kernel.
- Re-compile your kernel with your previous .config file and enable all the new options for ACPI.
That should be it.
May be there is a shorter route to use ACPI, but this one is the only one I know.
ACPI will allow you to do everything power related, shutwown, battery level, etc.. but also fan speed, cpu temperature, etc...
Hope it helps.