define in /etc/portsnap.conf

# Default location of the ports tree (target for "update" and "extract").
# PORTSDIR=/usr/ports
PORTSDIR=/usr/jails/ports

set variable in /etc/login.conf

:setenv=MAIL=/var/mail/$,BLOCKSIZE=K,PORTSDIR=/usr/jails/ports:\

next rebuild db as root

# cap_mkdb /etc/login.conf