EagerLoadProducts.cs

var order = db.GetCollection<Order>().Find(new { })
                                     .Include(o => o.ProductsOrdered)
                                     .First();

Order.cs

public class Order
{
        public Order()
        {
            Id = ObjectId.NewObjectId();
        }

        public ObjectId Id { get; set; }
        public string Name { get; set; }
        public DbReference<Product>[] ProductsOrdered { get; set; }
}

OrderMap.cs

public class OrderMap : MongoConfigurationMap
{
        public OrderMap()
        {
            For<Order>(o => o.ForProperty(p => p.ProductsOrdered)
                             .UseDocument("products"));
        }
}

RetrieveSample.cs

var order = db.GetCollection<Order>().Find(new { }).First();
var product = order.ProductsOrdered[0].Fetch(() => db);