If you need a serial number, you can find it at the place you bought the game. It should be listed on the download page, possibly emailed to you, or you should be able to log in and look it up.
If you are looking for cracks, you came to the wrong place.
Do you have the original release of D2:ED, or DKS (the remastered D2:ED and expansion FoV together)?
If the latter, the copy protection was removed, so you don't need a serial number. See: Dragon Knight Saga DRM free patch
If the former, you will need to email email@example.com. See: Serial number problem
(DKS topic, but I assume the procedure is the same for D2:ED).