From LedHed's Wiki
If you install Samba 3.x on FreeBSD 8.0 through 8.2 with the "ADS" config option the install will fail.
To get around this, install in the following order:
Install
Install Kerberos5
cd /usr/ports/security/krb5 make install
Install Samba 3.x
cd /usr/ports/net/samba3.5 make KRB5_HOME=/usr/local make install
NOTE: the extra step: make KRB5_HOME=/usr/local
References
http://lists.freebsd.org/pipermail/freebsd-net/2007-October/015567.html