Change it to void.
Code:
public static void makeConnection()
        {
            if(frmMain.con != null)
            {
                frmMain.con = new SqlConnection();
                frmMain.con.ConnectionString = "Data Source=.\\SQLEXPRESS;AttachDbFilename=C:\\MyMDB Records\\myMDB.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True";
            }
        }