Install FreeBSD kernel source files, but i have already installed freebsd.
The right way to manual installed kernel source:
Download the tar file that matches the version you installed.
i386: ftp://ftp.freebsd.org/pub/FreeBSD/releases/i386/9.1-RELEASE/src.txz
amd64:ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/9.1-RELEASE/src.txz
amd64:ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/11.0-RELEASE/src.txz
fetch ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/9.1-RELEASE/src.txz
Unpack the archive to /usr/src:
tar -C / -xvzf src.txz