you enable it in the PlaceOrder call, Transmit - boolean flag that specifies if given order should be actually transmitted to the exchange.
if this flag is set to FALSE then order is NOT transmitted but appears in the TWS workstation so you can press transmit it manually later, AND
you unlock automatic transmit in the Unlock code dialog (File->Enter Unlock code). IBController by default does not transmit orders automatically. Transmission can be enabled by entering unlock code that is made available to the user after accepting the agreement.