2004 Buick Century Remote Door Access

If your talking about a transmitter, you need a scan tool to reprogram it.

Did any of these methods work? Here is the information that I got for your vehicle.
Service Information
2004 Buick Century | Century, Regal ( VIN W) Service Manual | Document ID: 682874

Remote Control Door Lock Transmitter Programming
Important: All transmitters which are to be recognized by the remote control door lock receiver (RCDLR) must be programmed in a single programming sequence. If the system is placed in program mode it will erase all previously programmed transmitters upon the receipt of the programming signal from the first transmitter. Up to 4 transmitters can be programmed. Do not operate or program the transmitters in the vicinity of other vehicles that are in the keyless entry program mode. This prevents the programming of the transmitters to the incorrect vehicle.

Install a scan tool.
Use the scan tool in order to access programming: Vehicle Control System/Computer Integrating System/Module Setup/BCM/Program Key Fobs.
Press the START soft key on the scan tool.
Important: The scan tool must stay connected until all of the transmitters are programmed.

Press and hold the LOCK button and the UNLOCK button simultaneously on one transmitter. After a delay of 5-10 seconds, the scan tool will indicate that the transmitter is programmed.
Repeat the previous step in order to program up to 4 transmitters.
Remove the scan tool.
Operate the transmitter functions in order to verify correct system operation.

This is the information from ALLDATA on a 2004 century, it looks as though it is taken from the pages of GM service info.
TRANSMITTER PROGRAMMING

IMPORTANT: All transmitters which are to be recognized by the remote control door lock receiver (RCDLR) must be programmed in a single programming sequence. If the system is placed in program mode it will erase all previously programmed transmitters upon the receipt of the programming signal from the first transmitter. Up to 4 transmitters can be programmed. Do not operate or program the transmitters in the vicinity of other vehicles that are in the keyless entry program mode. This prevents the programming of the transmitters to the incorrect vehicle.

Install a scan tool.
Turn ON the ignition, with the engine OFF.
Use the scan tool in order to access the Body Control Module (BCM) Special Function: Program Key Fobs.
Press the START soft key on the scan tool. IMPORTANT: The scan tool must stay connected until all of the transmitters are programmed.
Press and hold the LOCK button and the UNLOCK button simultaneously on one transmitter. After a delay of 5-10 seconds, the locks will cycle in order to confirm the programming of that transmitter.
Repeat the previous step in order to program up to 4 transmitters
Remove the scan tool.
Operate the transmitter functions in order to verify correct system operation.
TRANSMITTER PROGRAMMING

IMPORTANT: All transmitters which are to be recognized by the remote control door lock receiver (RCDLR) must be programmed in a single programming sequence. If the system is placed in program mode it will erase all previously programmed transmitters upon the receipt of the programming signal from the first transmitter. Up to 4 transmitters can be programmed. Do not operate or program the transmitters in the vicinity of other vehicles that are in the keyless entry program mode. This prevents the programming of the transmitters to the incorrect vehicle.

This is the ALLDATA info for a 2005 Buick Century

Install a scan tool.
Use the scan tool in order to access the Body Control Module (BCM) Special Function: Program Key Fobs.
Press the START soft key on the scan tool.
Press and hold the LOCK button and the UNLOCK button simultaneously on one transmitter. After a delay of 5-10 seconds, the scan tool will indicate that the transmitter is programmed. IMPORTANT: The scan tool must stay connected until all of the transmitters are programmed.
Repeat the previous step in order to program up to 4 transmitters.
Remove the scan tool.
Operate the transmitter functions in order to verify correct system operation.

As you can see from my point, it is not meant to discourage you from performing your own repairs. There are some things that you may not be able to perform, and that is becoming more of the standard of operations as time goes by. A locksmith may have a different procedure that works, it is there perrogative to find alternate methods for them to remain competitive. I can only offer the information that I am provided.