Refactoring fixed static adding items to new List
Well, we can cut the boilerplate down a little bit with some helpers and changes to your original classes like this:

using System.Collections.Generic;

using static AdPosition;
using static AdType;

public class AdItem
{
    public AdType Type { get; set; }

    public string Container { get; set; }

    public SizeList Sizes { get; set; }

    public PositionList Position { get; set; }
}

public sealed class AdItemList : List&lt;AdItem&gt;
{
    public void Add(AdType type, string container, SizeList sizes, PositionList position)
    {
        this.Add(new AdItem { Type = type, Container = container, Sizes = sizes, Position = position });
    }
}

public sealed class SizeList : List&lt;List&lt;int&gt;&gt;
{
    public void Add(params int[] values)
    {
        var intList = new List&lt;int&gt;(values.Length);

        foreach (var value in values)
        {
            intList.Add(value);
        }

        this.Add(intList);
    }
}

public sealed class PositionList : List&lt;Position&gt;
{
    public void Add(string id, string value)
    {
        this.Add(new Position { Id = id, Value = value });
    }
}


then your method shrinks up like so:

private static AdItemList GetAdSlotItems()
{
    var s = "";
    return new AdItemList
    {
         { LeftSky, "div-ad-skyscraper-l", new SizeList { { 120, 600 }, { 160, 600 } }, new PositionList { { LeftSky.ToString(), left.ToString() } } },
         { RightSky, "div-ad-skyscraper-r", new SizeList { { 120, 600 }, { 160, 600 } }, new PositionList { { RightSky.ToString(), right.ToString() } } },
         { LeaderBoard, "div-ad-leaderboard", new SizeList { { 300, 250 } }, new PositionList { { inline.ToString(), inline.ToString() } } },
         { BillBoard, "div-ad-billboard", new SizeList { { 728, 90 }, { 970, 250 }, { 300, 50 } }, new PositionList() },
         { MPU1, "div-ad-rectangle-e1", new SizeList { { 300, 250 }, { 300, 600 } }, new PositionList { { MPU1.ToString(),mpu1.ToString() } } },
         { MPU1, "div-ad-rectangle-e2", new SizeList { { 300, 250 }, { 300, 600 } }, new PositionList { { MPU2.ToString(),mpu2.ToString() } } },
         { MPU1, "div-ad-rectangle-e3", new SizeList { { 300, 250 }, { 300, 600 } }, new PositionList { { MPU3.ToString(),mpu3.ToString() } } },
         { WallPaper, "div-ad-wallpaper", new SizeList { { 1, 1 } }, new PositionList() }
    };
}