The Unit Test does not see my methods

I created a unit Test project for my CalculatorApp and added the app as a reference for my UniTest Project

My code looks like this:

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using CalculatorApp;


    namespace CalculatorUnitTests
    {
        [TestClass]
        public class UnitTest1
        {
            [TestMethod]
            public void TestAddMethod()
            {
                decimal firstTestNumber = 7;
                decimal secondTestNumber = 3;
                decimal resultTest = 10;


                Assert.AreEqual(resultTest, AddMethod(firstTestNumber, secondTestNumber));

            }
    }

But it says that AddMethod does not exist in the current context.

What am I missing? (i don't have much experience with unitTests) I am using Visual Studio Community 2015.