Quick and dirty menu item.

gistfile1.txt

public override bool OnCreateOptionsMenu(IMenu menu)
{
    var item = menu.Add(0,1,1,"Back");
    //If you want an icon set it here
    item.SetIcon(Resource.drawable.ic_menu_back);
    return true;
}

public override bool OnOptionsItemSelected(IMenuItem item)
{
    switch (item.ItemId)
    {
        case 1: Back();
            return true;
        default:
            return base.OnOptionsItemSelected(item);
    }
}