You need a flashcart such as an M3 DS Simply or a Supercard. You take the memory card from your flashcart, which has your Pokemon Diamond ROM and save file on it. You insert it into the computer using a reader and then open this using the open button the save editor. For example it could be in E: and then using the editor itself is quite self explanatory.

Ask questions if you want any more help, I´ll happily answer them