Error acessing controller.... Asp.net mvc 4

Unless you specify ActionName attribute both actions will be found when "Admin" action is specified. Arguments are not taken into account when matching method to action name.

You can also use HttpGet/HttpPost attributes to specify one to be for GET and another for POST.

 [ActionName("AdminById")]
 public ActionResult Admin(int id)

And in route specify "AdminById" when path contains id.

As you have define two action method with same name,it get confuse about which method to call.
so of you put request first time and in controller you have two method with same name than it will show error like you are currently getting due to it try to find method with attribute HttpGet,but you have not mention that attribute on action method,now when you post your form at that time it will try to find method with HttpPost attribute and run that method,so you have to specify this two attribute on same method name  
    Try this
    [HttpGet]
    public ActionResult Admin()
        {
            var aux=db.UserMessages.ToList();

            return View(aux);         

        }
    [HttpPost]
        public ActionResult Admin(int id)
        {
            var aux = db.UserMessages.ToList();

            return View(aux);

        }

When a user views a page, that's a GET request and when a user submits a form, that's usually a POST request. HttpGet and HttpPost restrict an action method so that the method handles only corresponding requests.

   [HttpGet]
    public ActionResult Admin()
    {
        var aux=db.UserMessages.ToList();

        return View(aux);         

    }

    [HttpPost]
    public ActionResult Admin(int id)
    {
        var aux = db.UserMessages.ToList();

        return View(aux);

    }

In your case if you want to have a get request to the second method, you better rename your method.

In ASP.NET MVC you cannot have two actions with same name and same verb. You can write the code like this to keep your code readability.

private ActionResult Admin()
{
    var aux=db.UserMessages.ToList();
    return View(aux);         

}

public ActionResult Admin(int id = 0)
{
    if (id == 0)
        return Admin();

    var aux = db.UserMessages.ToList();
    return View(aux);

}