Ok, so I got my main AddIn which auto generated (Attachment 1)
and there is my Ribbon (Attachment 2) and I want to access the currently active excel sheet from that ribbon. But System.Windows.Forms.Application does not contain a definition for ActiveSheet.
Attachment 1:
using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml.Linq;
using Excel = Microsoft.Office.Interop.Excel;
using Office = Microsoft.Office.Core;
using Microsoft.Office.Tools.Excel;
using System.Windows.Forms;
namespace ExcelAddIn1
{
public partial class ThisAddIn
{
    private void ThisAddIn_Startup(object sender, System.EventArgs e)
    {
    }
    private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
    {
    }
    #region VSTO generated code
    /// <summary>
    /// Required method for Designer support - do not modify
    /// the contents of this method with the code editor.
    /// </summary>
    private void InternalStartup()
    {
        this.Startup += new System.EventHandler(ThisAddIn_Startup);
        this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
    }
    public void doStuff()
    {
       
    }
    
    #endregion
}
}
Attachement 2:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.Office.Tools.Ribbon;
using System.Windows.Forms;
using Excel = Microsoft.Office.Interop.Excel;
using Office = Microsoft.Office.Core;
using Microsoft.Office.Tools.Excel;
using System.Windows.Forms;
namespace ExcelAddIn1
{
public partial class Ribbon1
{
    private void Ribbon1_Load(object sender, RibbonUIEventArgs e)
    {
    }
    private void button1_Click(object sender, RibbonControlEventArgs e)
    {
        MessageBox.Show("Test");
        Excel.Worksheet activeWorksheet = ((Excel.Worksheet)Application.ActiveSheet);
        Excel.Range firstRow = activeWorksheet.get_Range("A1");
        firstRow.EntireRow.Insert(Excel.XlInsertShiftDirection.xlShiftDown);
        Excel.Range newFirstRow = activeWorksheet.get_Range("A1");
        newFirstRow.Value2 = "This text was added by using code";
    }
}
}
 
     
     
    