Hi there and welcome to the Forums!
As you have a black screen and a blinky cursor, I suspect that you may not have set the BIOS boot order to boot off the USB as its first choice. So, remove the USB stick, power up the computer and make the appropriate BIOS selections to put the USB first in the boot order. Assuming that you can see the M2 SSD in the BIOS, make that the second item in the boot order list.
Then, as I know nothing about Unetbootin, I take the easy way out and suggest that you go to the manual
here http://doc.freenas.org/11/install.html#on-freebsd-or-linux and follow the instructions to prepare the USB you will use to boot from and load FreeNAS onto your M2 SSD. It's important that the instructions are followed to the letter as the boot process can be "picky".
Then boot the target computer from your carefully prepared USB stick and follow the instructions in the user guide Section 2.3 to transfer the FreeNAS OS to the M2 SSD.
If it doesn't boot into the install menu then assume either you don't have the boot order set correctly in the BIOS or the USB stick is not properly prepared.
If you get stuck, come back and describe what you have done and what you can see on the monitor attached to the soon-to-be server.
Good luck!