Game is a Spigot plugin. That means when you are creating a game on OMGPI, you need to create a plugin.yml.
First, in your IDE import OMGPI same way as you import Spigot. It would be also very useful to use sources sometimes. Next, create a plugin.yml just as if you were making a Spigot plugin. Next we come to the main class:
public class YourMainClass extends Game {
  public onEnable() {
    name = "YourGameDisplayName";
The first difference you notice - main class extends Game instead of JavaPlugin. It is important for OMGPI since inside the onEnable we call super.onEnable(), which loads all OMGPI classes. But before calling the super, first of all set your game display name - it will be used by many of OMGPI systems, for example in VoteSystem it is shown in scoreboard for players.