Ok, I think we all assumed you were referring to the way ScreenOS did interface nat, which is src-nat.
Did you configure proxy arp, to map the interface arp to the hidden host on the other side of the firewall?
Can you give more details about your requirement? Are you looking to nat incoming connections to a protected server? Perhaps a network topology and what it is you are trying to accomplish will clear up things.
I assume you followed the app note on nat,
http://www.juniper.net/us/en/local/pdf/app-notes/3500152-en.pdf (destination nat is near the end of the document)?