We need more information what exactly you want to do.
From what I understand you have Pokemon on a Diamond save on your computer (DeSmuMe) and you'd like to transfer the Pokemon to your Black game (unsure if it is a ROM on a flashcard or a retail version).
If you have a flashcard with your Black ROM, then you can plug in your MicroSD and I can show you how to edit the save file to inject your Diamond Pokemon into the Black Save.
If you have a Black retail cart, then if you own a flashcard then you can extract the save and do the same as above. If you don't have a flashcard at all, you can send Pokemon through a GTS exploit to your retail Black.