On Linux/Unix platforms after downloading the Installer you must make it executable, by opening a shell, going into the downloads directory (via cd) and running the command chmod +x, for example: chmod +x CERTivity-unix-0.2.sh.
Then you start the installer either from a visual file manager or from the command line by running the above executable script in the same directory, for example: ./CERTivity-unix-0.2.sh.