Obtaining an API Instance

Just cast the plugin object to the API instance. For example:

(This is just an example, you should, of course, check to ensure the plugin is loaded.)

public class ExamplePlugin extends JavaPlugin {
    private IPacketEntityAPI instance;

    public void onEnable() {
        instance = (IPacketEntityAPI) Bukkit.getPluginManager().getPlugin("PacketEntityAPI");
    }
}