AddModelError(Model, x=>x.PropertyName, "text")

ModelStateDictionaryExtensions.cs

namespace System.Web.Mvc
{
    using Linq.Expressions;

    public static class ModelStateDictionaryExtensions
    {
        public static void AddModelError<TEntity, TValue>(this ModelStateDictionary modelState, TEntity model, Expression<Func<TEntity, TValue>> member, string value)
        {
            var memberPath = ExpressionHelper.GetExpressionText(member);
            modelState.AddModelError(memberPath, value);
        }
    }
}

Usage:
ModelState.AddModelError(Model, x => x.PropertyName, "Validation message")