private void exportBtn_Click(object sender, EventArgs e)
{
    OleDbCommand newRow = new OleDbCommand();
    newRow.CommandType = CommandType.Text;
    newRow.CommandText = "INSERT INTO [Sheet1$] ([Company:], [County:], [State:], [Notes:], [User:], [Email:], [Username:], [Password:], [SMMM PWD:], [CAD:]) VALUES (1,2,3,4,5,6,7,8,9,10)";
    newRow.Connection = connectionExl;

    connectionExl.Open();
    newRow.ExecuteNonQuery();
    connectionExl.Close();
}

public int getLastRow(Excel.Worksheet ws)
{
    object[,] arData = ws.UsedRange.FormulaR1C1; // FormulaR1C1 returns only string in 2 dimensional array
    for (int iRow = arData.GetUpperBound(0); iRow > 0; iRow--) {
        for (int iCol = 1; iCol <= arData.GetUpperBound(1); iCol++) {
            if (arData[iRow, iCol].ToString() != "") { return iRow; }
        }
    }
    return 0;
}

int lastRow = ws.Cells.SpecialCells(Excel.XlCellType.xlCellTypeLastCell).Row;

using System.Runtime.InteropServices;
using Excel = Microsoft.Office.Interop.Excel;

void test()
{
    Excel.Application xlApp = csXL.StartExcel();
    Excel.Worksheet ws = xlApp.ActiveSheet;
    int irow = getLastRow(ws);
}

public static Excel.Application StartExcel()
{
    Excel.Application instance = null;
    try { instance = (Excel.Application)Marshal.GetActiveObject("Excel.Application"); }
    catch (System.Runtime.InteropServices.COMException ex) { instance = new Excel.Application(); }
    return instance;
}

object[,] arLoad = new object[0, 9]; arLoad[0, 0] = 1; arLoad[0, 1] = 2;
ws.Range["A" + irow + ":J" + irow].FormulaR1C1 = arLoad; // or use Value instead of FormulaR1C1