Are you trying to access a printer over a remote desktop / terminal server link or from a virtual machine? As I believe that isn't supported in the SDK, it has to be on the connected directly via usb or via an ethernet connection.

If the code

             object _deviceList = null;
            ZXPPRINTERLib.Job job = new ZXPPRINTERLib.Job();
            try
            {
                job.GetPrinters(ZXPPRINTERLib.ConnectionTypeEnum.All, out _deviceList);
            }
            catch
            {
                _deviceList = null;
            }

            j.Close();
            do
                    {
                        Thread.Sleep(10);
                    } while (Marshal.FinalReleaseComObject(j) > 0);

doesn't find it then it won't work.