C:\Program Files (x86)\Steam\steamapps\common\Cities_Skylines\Files\Mods\mod_name\Source

EnableAchievements.cs

using ICities;
using ColossalFramework;

namespace SkylinesMod
{
    public class EnableAchievements : IUserMod
    {
        public string Name
        {
            get { return "Enable Achievements"; }
        }

        public string Description
        {
            get { return "Enables achievements when mods are loaded."; }
        }
    }

    public class EnableAchievementsLoad : LoadingExtensionBase
    {
        public override void OnLevelLoaded(LoadMode mode)
        {
            Singleton<SimulationManager>.instance.m_metaData.m_disableAchievements = SimulationMetaData.MetaBool.False;
        }
    }
}