There is a current build for the SX620 HS 100c — the port is listed as stable with firmware 100b and 100c, so nothing about your camera is stuck at 1.3. What you have is an old download. Get the current build from build.chdk.photos (or the /cameras list) and install the full package, not the small one: the curve files live in the CURVES folder, which only the full package contains, so a small-package install is the usual reason the curve tool shows up empty or missing.
Since you already have a working 1.3 install, replace the whole CHDK folder rather than merging, so old files don't linger.
