Are you sure you are entering the correct installation directory?
You don't need the installer script. Just follow the instructions in Readme.txt:
To manually install the Plug-in Player for Linux, follow these directions:
- Copy the following files to your browser's plugins folder:
The "plugins folder" means the subdirectory plugins
under the directory where Mozilla is installed in. You should find at least libnullplugin.so (or something similar) there. Alternatively, you can copy the plug-in to your home directory so only one user can use it. (There is a hidden directory called .mozilla for storing your preferences.)
Close the browser and start it again. The plug-in should work now.