I added a ZIL device (120GB SSD) to 2 different pools – if you only have 1 pool (or more) obviously change the directions below. AND my SSD showed up as da0, yours may be different!

Destroy the current file system on your SSD (if this command fails, move onto the next):
gpart destroy -F da0

Create GPT filesystem (if this command fails, try the previous command):
gpart create -s GPT da0

Add a partition
gpart add -s 64G -t freebsd-zfs da0
(da0p1 added)

Add a second partition (optional)
gpart add -s 32G -t freebsd-zfs da0
(da0p2 added)

Verify the partitions
gpart show da0

Add the ZIL to the appropriate pool
zpool add POOLNAME log da0p1
zpool add POOLNAME2 log da0p1

Optional: Remove the pool device
zpool remove POOLNAME DEVICENAME

Check ZIL Usage
zpool list -v
zpool iostat -v 2 300
gstat -a

***EDIT 7/25/14***
I also wanted to add a RAID10 array to my ZFS. I have a supermicro 4U chassis with 24x 3.5″ hot swap sata/sas drives with an Areca 1882i controller. I have 2x SSD in bay 0 and 1 for the ZIL, then 22x 3.5″ 3TB SATA drives. I wanted to have 2 hot spares.

zpool create -m /mnt/zfs zfs mirror da3 da4 mirror da5 da6 mirror da7 da8 mirror da9 da10 mirror da11 da12 mirror da13 da14 mirror da15 da16 mirror da17 da18 mirror da19 da20 spare da21 da22

zpool status

On nas4free I had to go to Disks|zfs|configuration|sync to have it show up in my disks to manage.

zpool set autoreplace=on POOLNAME