The PLATFORMSUB variable is the firmware version (here is explained how to know it for your camera:
FAQ - CHDK Wiki).
However you should not ADD new variables in makefile.inc, because if it's not there it means that CHDK hasn't been ported to this platform. But A720 and A720IS seems to be the same, so try with PLATFORM=a720 which is in makefile.inc, check your firmware version and hope it is PLATFORMSUB=100c...
EDIT: And please don't post the same question at different places in the forum... (continue the original thread if you want more precisions)
A newbie helpHello all