Actually making the plugin add working actions and API functions for ReaScript to use is quite involved. You for example have to implement the action handling callback yourself. Adding API functions requires lots of obscure boilerplate code per added function too. But I'll try come up with some examples. (You probably by the way meant the "hello world" action to actually print something on the Reaper console or show a message box instead of returning a string that isn't going to go anywhere...?)
edit : I made a new "minimal" example here that adds some actions (no API functions yet, doing that is so obscure I'll need to study again how that is done...You could look at Julian Sander's extension plugin for inspiration on that, though) :
https://github.com/Xenakios/reaper_very_minimal