Unless the policy has changed, the first step is that you need to call and opt yourself in to get yourself on the upgrade list. You don't just get to choose a MCS select seat just because you are Silver/Gold. Once you are cleared and confirmed to MCS during the upgrade window (within 12 hours of flight time for silver) you can then select a seat. You can only select the Express seats in main cabin more than a day in advance of the flight.