Zenwalk booting is controlled by Grub
Grub is controlled by the configuration file /boot/grub/menu.lst
To find the partition that has menu,lst in a Grub prompt
The response should by something like (hdi,j) meaning the j+1 parition in the i+1 disk as Grub counts from 0. So 1st partition of 1st disk is (hd0,0). Say Grub reports menu.lst is in (hd0,1) you can boot Zenwalk up by activate its menu.lst
Code:
configfile (hd0,1)/boot/grub/menu.lst