If you're using Booter 1.1 or later, you can just hit 'a' at the boot
screen.

If you're using an older Booter (some people can't use the newer one),
you'll need to edit the lilo.conf file in the Preferences Folder
(Mac-side) and add the line:

mach_options=-a

Culled from a post by David A. Gatwood to
mklinux-setup by jonh@cs.dartmouth.edu.