Here's a free code snippet that will return the path to the folder, or a null string if the user cancels.

public static String GetFolder()
{
   System.Windows.Forms.FolderBrowserDialog fbd = new System.Windows.Forms.FolderBrowserDialog();
   DialogResult result = fbd.ShowDialog();
   if (result == System.Windows.Forms.DialogResult.OK)
   {
      return fbd.SelectedPath;
   }
   return null;
}