Action Replay
Using an Action Replay is fairly simple. It has a cartridge slot, similar to the Nintendo DS cartridge slot, that is used to connect to the game. By inserting "Pokemon Diamond" into the cartridge slot of the Action Replay, the connection is made, and the Action Replay then can be inserted into the DS slot like another game. Once the Nintendo DS is turned on, you input any codes you wish to use in the game.
Infinite Money
By inputting a code into the Action Replay, you can have infinite money. You can then use that money to purchase as many Pokeballs as you wish. The code is:
94000130 fcff0000
B21c4d28 00000000
B0000004 00000000
000002e4 000f423f
D2000000 00000000
It can be inserted at the Action Replay code screen. To activate the code in the game, simply hold the "L" and "R" buttons.
Pokeballs Code
By inputting the following code at the Action Replay code screen, you can receive 999 Pokeballs:
94000130 fcff0000
b21c4d28 00000000
b0000004 00000000
00000f4c 03e70001
d2000000 00000000
As with the previous code, you simply press and hold the "R" and "L" buttons to activate the code. You can use this code as many times as you wish to receive 999 Pokeballs.
Warnings
Using an Action Replay to hack or modify your game save file can result in your file being corrupted. Many players have even reported that their entire save file was erased from using an Action Replay device, as it tampers with the programming of "Pokemon Diamond." In addition, you cannot use any game that has been hacked by an Action Replay in any Pokemon tournament sponsored by Nintendo.