The Heathen Engineering Steamworks integration is an easy way to get Steamworks.NET working in Unity without code, and so I figured that some people might be using it to set up Steamworks in their project like I am. However, this means that the Achievement Unlock custom action by @Alverik will no longer work correctly. So, I've made a few edits to the script to correctly work with the Heathen Engineering Steamworks, which you can view below.
Hope this helps some people!

using UnityEngine;
using System;
using HeathenEngineering.SteamworksIntegration;
using HeathenEngineering.SteamworksIntegration.API;
using StatsClient = HeathenEngineering.SteamworksIntegration.API.StatsAndAchievements.Client;
using Steamworks;

#if UNITY_EDITOR
using UnityEditor;
#endif
/* By Alvaro Urrutia Luna (Alverik)
* Edited by Ryan Zollinger (Dr. PUZZLER) to work with Heathen Engineering Steamworks
* Custom action to
* Unlock Steam achievements.
*/
namespace AC
{

    [System.Serializable]
    public class ActionUnlockAchievement : Action
    {

        public string AchievementID  = "";

        public override ActionCategory Category { get { return ActionCategory.Custom; }}
        public override string Title { get { return "Unlock Steam Achievement."; }}
        public override string Description { get { return "Unlocks an steam Achievement."; }}

        override public float Run ()

        {
            //your action:
            if (SteamSettings.Initialized) 
            {
                if(AchievementID!=String.Empty || AchievementID!="")
                {
                    try
                    {
                        StatsAndAchievements.Client.SetAchievement(AchievementID);
                        StatsAndAchievements.Client.StoreStats();
                    }
                    catch(Exception e)
                    {
                        Debug.LogError("Error while setting steam achievements... " + e);
                    }

                }
            }
            //end the action
            return 0f;
        }


        #if UNITY_EDITOR

        override public void ShowGUI ()
        {
            // Action-specific Inspector GUI code here
            AchievementID  = EditorGUILayout.TextField("Achievement ID: ", AchievementID , GUILayout.ExpandWidth(true));
        }


        public override string SetLabel ()
        {
            // Return a string used to describe the specific action's job.
            string labelAdd = "Unlocks an steam Achievement.";
            return labelAdd;
        }

        #endif

    }

}