If you've done your castle quests correctly, you should be able to get rid of the transmitter, which gets replaced and should give you a new D-03 I believe.
This quest here http://maplearchive.com/quest-wp.php?id=31435 tells me that that should be around the forest part. Where you lose the original D-03 and get a crappy one. Using it at the right location after you activate a certain event in there should probably trigger it. Can you even go to all the bosses besides Magnus?
This quest here http://maplearchive.com/quest-wp.php?id=31435 tells me that that should be around the forest part. Where you lose the original D-03 and get a crappy one. Using it at the right location after you activate a certain event in there should probably trigger it. Can you even go to all the bosses besides Magnus?