Here is the address to write to Ed Kemper:
Edmund Kemper, B52453
California Medical Facility
P.O. Box 2000
Vacaville, California 95696-2000
USA
Here is more info on how to send your letter or card to him:
http://www.prisonpro.com/content/california-inmate-phonessending-money-mail
He doesn’t answer much mail anymore apparently.