To make something executable, just open a file manager, "right-click" it and select "properties", select the "permissions" tab and check the box that says "allow executing file as program"
Also, to install the plugin, you just need to copy the extension.py file into the right folder. That's it, gimp will see it the next time you open it.
The folder is hidden in your home directory. Press "ctrl +h" to show hidden files and navigate to: /home/user/.gimp-2.8/plug-ins/
Just copy your plugins in here.