1. Download Free_BSD_Driver_P20 from:

http://www.lsi.com/support/pages/download-results.aspx?component=Storage+Component&productfamily=Host+Bus+Adapters&productcode=P00049&assettype=Driver&productname=LSI+SAS+9211-8i

1. Dig through the archive and extract mpslsi.ko from the following tree:

Free_BSD_Driver_P20\MPS_LSI_Phase20.0-20.00.00.00\FREEBSD_MPS_SAS2.tar.gz\FREEBSD_MPS_SAS2.tar\pkg\stage\9.3.0\amd64\boot\kernel

1. Copy it to the FreeNAS system boot\modules or boot\kernel (I copied to both, as I'm not sure which one is actually correct).
2. Add a new tunable in FreeNAS:

Variable: mpslsi_load
Value: YES
Type: Loader
Comment: LSI P20 Driver
Enabled [Ticked]

Finally, reboot.