When executing an executable or script that isn't in your system's "PATH" environment variable, you have to provide the full path to it. "." is an alias for the full path of your current working directory, so ./install_zs.sh 5.3
should work.
I'd like to strongly suggest you give this, or some similar *nix shell documentation a read-through, as maneuvering around the shell blindly is never, ever, ever a good idea.