Apply by doing:
       cd /usr/src
       patch -p0 < 006_nat-t.patch

And then rebuild and install isakmpd:
       cd sbin/isakmpd
       make obj
       make depend
       make && make install

Index: sbin/isakmpd/nat_traversal.c
===================================================================
RCS file: /cvs/src/sbin/isakmpd/nat_traversal.c,v
retrieving revision 1.10
diff -u -p -r1.10 nat_traversal.c
--- sbin/isakmpd/nat_traversal.c        5 Mar 2005 12:22:40 -0000       1.10
+++ sbin/isakmpd/nat_traversal.c        5 Nov 2005 21:37:37 -0000
@@ -58,8 +58,7 @@
 */
static const char *isakmp_nat_t_cap_text[] = {
       "draft-ietf-ipsec-nat-t-ike-02\n",      /* draft, V2 */
-       "draft-ietf-ipsec-nat-t-ike-03",        /* draft, V3 */
-       "RFC 3947"
+       "draft-ietf-ipsec-nat-t-ike-03"         /* draft, V3 */
};

/* In seconds. Recommended in draft-ietf-ipsec-udp-encaps-09.  */