Welcome to the Horde
Please only run one WTTH or similar per game. Check to see if your game already has one before starting your own. Nothing is gained by running duplicates, and it wastes the resources of Twitter and HVZ Source.
WTTH now has a gem in Gemcutter and can be installed as wtth. This should install all the dependencies and put wtth into your path like magic. If you'd previously installed mdonoughe-wtth from Github's gem service, you should uninstall that version before installing the new one.
After you've gotten everything installed, run wtth init to give the program access to a Twitter account. I'd suggest not using your usual Twitter account for this.
Running wtth init will create a file called .wtth.yml in your home directory. You'll want to open that and change player_list_uri to point at the correct HVZ Source page for your game.
You'll probably want to make some sort of cron job to run wtth. It makes at most one request to HVZ Source and posts at most one message to Twitter per run. Set this to a responsible interval. You can probably get away with as low as once a minute, but that's completely unnecessary in my opinion.
Please remember not to have the cron job running when the game is not in session.