You might try running the Reset PixInsight Settings app.
You might also check your PI install directory to see if the script still appears. On Win7 it is located here:
PixInsight/src/scripts/mschuster/SubframeSelector/
This directory should contain 14 .js file and 3 .txt files.
If any of this is missing, you may want to try deinstalling and reinstalling PI.
Thanks,
Mike