BurgZergArcade Script Error

I'm following the tutorials of a Hack and Slash game on BurgZertgArcade.com I thinks i've done everything correctly, but when I want to lay, an error appears on the console that says: Assets/Scripts/Player/CharacterClasses/ModifiedStat.cs(25,33): error CS0019: Operator `+=' cannot be applied to operands of type`System.Collections.Generic.List' and `int'.

What can I do? What have I done wrong please help!

using System.Collections.Generic;
public class ModifiedStat : BaseStat
{
    private List<ModifyingAttribute> _mods;         //A list of attributes taht modify the stat
    private int _modValue;          //The amount added to the base value of the modifiers

    public ModifiedStat()
    {
        _mods = new List<ModifyingAttribute>();
        _modValue = 0;
    }

    public void AddModifier( ModifyingAttribute mod)
    {
         _mods.Add(mod);
    }

    private void CalculateModValue()
    {
        _modValue = 0;

        if(_mods.Count > 0)
        {
            foreach(ModifyingAttribute att in _mods)
                _mods += (int)(att.attribute.AdjustedBaseValue * att.ratio);
        }

    }

    public new int AdjustedbaseValue
    {
        get{ return BaseValue + BuffValue + _modValue; }

    }

    public void Update()
    {
        CalculateModValue();
    }

}

public struct ModifyingAttribute
{
    public Attribute attribute;
    public float ratio;

    public ModifyingAttribute(Attribute att, float rat)
    {
        attribute = att;
        ratio = rat;
    }
}

funny enouth i working on a problem on the same scipt from the same tutorial, but have a

Assets/scripts/character Classes/ModifiedStat.cs(38,35): error CS0118: `ModifyingAttribute.attribute' is a`field' but a `type' was expected

error instead.

here is my script :-

    using System.Collections.Generic;

public class ModifiedStat : BaseStat {
    public List<ModifyingAttribute> _mods;     // a list of attributrsd that modify this stat
    public int _modValue;      // the amout added to the baseValue from the modifyers.

    public ModifiedStat() {
        _mods = new List<ModifyingAttribute>();
        _modValue = 0;
    }

    public void AddModifier( ModifyingAttribute mod) {
        _mods.Add(mod);
    }

    public void CalculateModValue() {

        _modValue = 0;

        if(_mods.Count > 0)
            foreach(ModifyingAttribute att in _mods)
            _modValue += (int)(att.attribute.AdjustedBaseValue * att.ratio);
    }

    public new int AdjustedBaseValue {
        get { return BaseValue + BuffValue + _modValue; }
    }

    public void Update() {
        CalculateModValue();
    }
}

public struct ModifyingAttribute {
    public Attribute attribute;
    public float ratio;

    public ModifyingAttribute(attribute att, float rat) {
        attribute = att;
        ratio = rat;
    }
}

and i think i just remebered somthing, - relook at you BaseStat.cs script - i re watched vids 11 - 20 and fix the error i hade with your line, - but a diffent error which i fixed by re typeing stuff in basestat. i know i got a site problem and inless i pase a screen shot into a image viewer i cant make shore i following the tutorial.