Instructions
Insert the "Pokemon Platinum" game cartridge into the Action Replay, then load the device into your Nintendo DS. Turn on the system and let it load the Action Replay main menu.
Tap the "Codes" button to see a list of games associated with the Action Replay. If "Pokemon Platinum" is not in the list, tap "Add new game."
Type "Pokemon Platinum" as the game's name using the virtual keyboard. When you are finished, press "Start." On the next screen, you will be asked for the Game ID, but this will be automatically filled in by the Action Replay device, so press "Start" to go back to the games list.
Select "Pokemon Platinum" from the list. Tap the "Add new code" button. Name the code using the virtual keyboard. The code name should be something related to the code's effect on the game. For example, if you are cheating to get a Pokemon that would normally require a special event, use that Pokemon's name as the code. Press "Start" when you are finished.
Type in the code exactly as it is written in your source. When you are finished, press "Start" to save the new code on the Action Replay device.