This will create a file in the MyDocuments folder called userdata.text. File will be wrote to the Stopwatch Data folder, as I believe you were trying to do?
var dir = Path.Combine(Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments), "Stopwatch Data"));
if (!Directory.Exists(dir))
Directory.CreateDirectory(dir);
StringBuilder sb = new StringBuilder();
foreach (string Item in listBox2.Items)
{
sb.AppendLine(Item);
}
var file = Path.Combine(dir, "userdata.text");
File.WriteAllText(file, sb.ToString());