MEGA 2560 TX1/RX1 to USB Serial Adapter

I am trying to connect a MEAG 2560 tx1/rx1 to a PC Serial port through a USB to Serial adapter. I am looking for suggestions for getting the signals to the TTL levels required by the MEGA board? I am using Serial(0) to communicate with and control my hardware using a RS232 Shield but I need to be able to send some commands manually using a terminal program. I have my program runnig OK using input from the Arduino serial monitor but that ability goes away when I attach the shield. Thanks.

I am not understanding clearly. Provide some detail about the shield and board (which exactly). Do you want to use USB/serial adapter or RS232 shield for Serial1?
Anyway, it is probably problem with impedance load on Rx0/Tx0. Serial(0) is used by Arduino's USB and there is probably conflict with RS232 shield (which probably uses the same port 0) but the board has next 3 serial ports to be used same way. To use Serial1 you can try to wire rx1/tx1 with RS232 shield on the breadboard.
Or, if you used USB/serial adapter for rx1/tx1 why don't you use onboard USB and you are trying to use RS232 shield?