This is how i have mine: 

Model: 

[Required(ErrorMessage = "Whatever you would like.")]
        public bool? nameOfString { get; set; }

and then in the View:

@Html.DropDownListFor(x => x.ZNameOfString, new[] {
                    new SelectListItem() {Text = "WhateverYouWouldLike", Value = bool.TrueString},
                    new SelectListItem() {Text = "WhateverYouWouldLike", Value = bool.FalseString}
                }, "ThisIsWhatTheDefaultValueWillBe")

..Hope this helps