I click on the raw drive (SEAGATE1234) not the user partition, and the RAID tab is there.
I am not sure you want to ever mirror SSDs.
People do use stripe and people should always have a restore image of any SSD.
Yes you can have failures for which a restore image is first essential item.
Only then consider mirror, of like identical models of drives.
"Green" are not suitable for mirror or stripe, or even for most data storage if you want performance and don't want frequent head-parking and spin down that green drives entail.
I would format each on its own, test each to insure they work and they are identical in tests of IOPS and r/w behavior. Then drag them into the RAID window.
WD goes from 2TB to now 4TB but no 3TB in the Black model series.
I would use this for my data needs, and maybe use Green for backups, and you want something large for TimeMachine.
http://www.amazon.com/Black-Desktop-Hard-Drive-WD4001FAEX/dp/B00A2IM76K/