mau install kernel baru tapi ga punya source-nya, atasi dengan.. pertama download iso freebsd disk 1, lalu mount dengan perintah

mdconfig -a -t vnode -f 7.2-RELEASE-i386-disc1.iso  -u 0

lalu mountting

mount -t cd9660 /dev/md0 /mount_point

kemudian masuk ke directori source

cd /mount_point/7.2-RELEASE/src]

lalau jalankan install.sh

./install.sh 
You must specify which components of src to extract
possible subcomponents are:

base bin cddl contrib crypto etc games gnu include krb5
lib libexec release rescue sbin secure share sys tools ubin
usbin

You may also specify all to extract all subcomponents.

pilih, source yang mau di ekstrak, misalkan kernel maka jalankan

./install.sh sys
Extracting sources into /usr/src...
  Extracting source component: sys
Done extracting sources.

kalo sudah, monggo di cek..

ls /usr/src/sys/
Makefile	conf		gnu		net		netnatm		opencrypto	sun4v
amd64		contrib		i386		net80211	netncp		pc98		sys
arm		crypto		i4b		netatalk	netsmb		pccard		tools
boot		ddb		ia64		netgraph	nfs		pci		ufs
bsm		dev		isa		netinet		nfs4client	powerpc		vm
cam		fs		kern		netinet6	nfsclient	rpc		xdr
cddl		gdb		libkern		netipsec	nfsserver	security
compat		geom		modules		netipx		nlm		sparc64