Quick Howto:
Download the package fromhttp://www.lsi.com/storage_home/products_home/internal_raid/megaraid_sata/megaraid_sata_3008x/index.html
Extract on another machine, and take the megarc binary file out of it.
Put the megarc binary on a share on the FreeNas box
Enable ssh and root access
Log on as root on the FreeNas box
Go to the location where you put the megarc binary
Run
./megarc -SilenceAlarm -a0
Where -a0 denotes the adapter number
Silence!!!!