i just got back from hibernation.
how to unlock it, is there any quest?
Once you hit level 50 and completed the Epilogue area Story Quests, you'll get the Episode 58 quest to collect 5 boss drops from the E-4 boss. Then you have to do another quest to turn in 100 of the green DNA strands.
After that, you will unlock it and get PNA EXP from the end of any Epilogue or Plane Gate stage run.
The items for the PNA system can be obtained through quests, bought from the scientist girl, boss drops in Plane Gate stages, or off the Black Market since they are tradeable.