FreeBSD sysinstal utility does not support at this time installation to ZFS file systems on GPT-style partitioned disks.
The first thing is to start a shell on the system where the target disk is attached. I used the FreeBSD bootable DVD, using the 'Fixit Shell'. On my configuration, the disk is attached to a SCSI controller, so the disk is identified by 'da0'. When installing to an ATA disk, it may be named 'ad0' or 'ada0', depending on your configuration.
The disk must be made bootable, i need to install the boot loader on the partition with index 1
After creating partitions, it's time to begin working on ZFS pool and volumes
ZFS supports compression for file systems. When compression is active, there will be some latency while reading or writing to these filesystems, depending on the algorithm used. If you change this setting, it will take effect for the files written AFTER change.
Instruct the boot loader to load the required modules and where to look for the root volume
Set the mount points for ZFS filesystems. Because /usr filesystem will be remounted, it's needed to instruct the linker where to find libraries
The / filesystem will be mounted automatically at boot time. The other filesystems won't need explicit entries in /etc/fstab if the ZFS mountpoint option is set, otherwise they must be listed in /etc/fstab and zfs mountpoint set to 'legacy' for these filesystems.
Remove the DVD (or USB key), reboot the system.
The others settings can be adjusted as usual, by using sysinstall or by hand.