[syslinux] Boot Multiple Kernels

[mzmrizu at houston.rr.com]

I can't seem to pick up on how to enter the path in the SYSLINUX.cfg file,
here is what I have tried:

label 3esx86
  kernel 3esx86/vmlinuz
  append initrd=3esx86/initrd.img

also tried

label 3esx86
  kernel /3esx86/vmlinuz
  append initrd=/3esx86/initrd.img

but I get errors like "cannot find kernel 3esx86.vm " or similar.

I am booting from a thumbdrive and it works fine if I point these statements
to a single kernel in the root, but I have different kernels in
directories /3esx86 and /4esx86 that I would like to boot from.  The
thumbdrive is a single FAT file system.  

Also, is there a limitation to the length a "label" can be?
Thanks,
Antonio