C# wrong reference to namespace

From what you described above and from my understanding, it seems that Managers and Models are two different projects. If that is the case, make sure that you add a reference to BusinessLogicLayer.Models in your BusinessLogicLayer.Managers.

If, on the other hand, you have only two projects BusinessLogicLayer and DataAccessLayer then it could very well mean that Library.BusinessLogicLayer.Models is not the name of the namespace.

UPDATE

From the picture that you added, you might need to add a reference to Library.BusinessLogicLayer.Models.Models. You have a folder named Models and a project named Models. Visual Studio automatically generates namespaces based on the Solution name, Solution folders, project name, folders within project.

There were three issues with your code. The first one is that you are supposed to add a reference to Library.DataAccessLayer.Models and not to Library.BusinessLogicLayer.Models. This is due to the fact that you have User in DataAccessLayer.Models and User2 in BusinessLogicLayer.Models.

The other two issues were with the Map method where you are sending incorrect number of arguments to the constructor (you are missing UserId) and the other issues is with your DateOfBirth and DateJoined being in the wrong order in the same method.

using System;
using System.Collections.Generic;
using System.Linq;
using global::Library.BusinessLogicLayer.Models;
using Library.BusinessLogicLayer.Managers.Properties;
using Library.DataAccessLayer.Models; // <-- Add reference to this

namespace Library.BusinessLogicLayer.Managers
{
    public class Users2
    {
        public IEnumerable<User> GetAll()
        {
            using(DataAccessLayer.DBAccess.Library library = new DataAccessLayer.DBAccess.Library(Settings.Default.LibraryDbConnection))
            {
                return library.Users.GetAll().Select(user => Map(user));
            }
        }

        private User Map(DataAccessLayer.Models.User dbUser)
        {
            if (dbUser == null)
                return null;

            // TODO: Constructor is missing a paremeter. I'll add a temporary one
            int tempUserId = 0;
            User user = new User(tempUserId, dbUser.Name, dbUser.UserName, dbUser.Password, dbUser.Email, dbUser.DateJoined, dbUser.DateOfBirth) // <-- The last two params are in the wrong order
            {
                Id = dbUser.Id
            };

            return user;
        }

        private Library.DataAccessLayer.Models.User Map(User2 user)
        {
            if (user == null)
                throw new ArgumentNullException("user","Valid user is mandatory!");

            return new DataAccessLayer.Models.User(user.Id,user.Name, user.UserName, user.Password, user.Email, user.DateJoined, user.DateOfBirth);
        }
    }
}

Also, regarding the last screenshot that you provided, you do not have Library.BusinessLogicLayer.Models2 namespace. Remove number 2 to get it to work.

As I don't have permission to update your repo with the fixed code, you'll have to fix it manually based on my answer. Otherwise, let me know so that we see how I can push the code back.