First we need to mount the zroot/ROOT/install dataset that we have created, I chose to use the /mnt mountpoint, like /tmp & /var/tmp is a memory backed disk on the live CD system.
Once that is mounted, change to the /mnt directory and use tar to extract the distribution files; base.txz, kernel.txz, and optionally lib32.txz. The first two are the base operating system, and kernel, the third is the 32 bit compatibility libraries. I personally recommend installing them, of course if you are building on a 32 bit system (not recommended with ZFS, but it is possible), they are not available.
mount -t zfs zroot/ROOT/install /mnt cd /mnt tar -xvpf /usr/freebsd-dist/base.txz tar -xvpf /usr/freebsd-dist/kernel.txz tar -xvpf /usr/freebsd-dist/lib32.txz
Note: -xvpf: -x tells tar to extract, -v tells it to be verbose, -p tells it to preserve permissions and ownership, -f tells it the following is a file to use as the source.