There is no such overload like your first example. You may try this overload
public static MvcHtmlString TextAreaFor<TModel, TProperty>(
this HtmlHelper<TModel> htmlHelper,
Expression<Func<TModel, TProperty>> expression,
int rows,
int columns,
Object htmlAttributes
)
So your example should be like this
@Html.TextAreaFor(model => model.Notes,5,10, new { @class= "myCssClassName" })
For your second example, you are using the overload in the correct way.