For the outbound traffic, you will create a dip for the desired address and a matching policy with the desired source and port addresses for the source nat.
See page 38 pdf printed page 20
http://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_ce_AddressTranslation.pdf
For inbound traffic destination nat you also need the policy to match the desired ports using many to one nat example.
see page 59 pdf printed page 41
http://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_ce_AddressTranslation.pdf