I had the same issue on FreeBSD 5.3...
I had to remove the / from the entry in the sendmail.cf
MAIL_FILTER(`mimedefang', `S=local:/var/spool/MIMEDefang/mimedefang.sock, F=T, T=C:15m;S:4m;R:4m;E:10m')dnl
You should only have 2 lines of code not 3
For your mimedefang... reread the page you will notice you have to modify one of the conf files to tell it to point and look for the correct mimedefang.sock