In the setup screen where it shows the available partitions, there's a button at the bottom that lets you add a driver, from there you should be able to add the driver for the controller that will allow the RAID volume to show up.
Couple of things to keep in mind:
Server 2012 is x64 only, and will only accept signed x64 drivers, and it may or may not accept the 2008 x64 or 2008R2 drivers even if they are signed.
(This article https://technet.microsoft.com/en-us/library/jj134246(v=ws.11).aspx lists the requirements for 2012, and it does show how to disable driver signing enforcement, which may be an option to get your setup to work, although it is not recommended)
You might give Server 2008 x64 or 2008R2 a shot, although since it went EOL in 2008 there may not be 2008R2 drivers for it. You might get lucky and be able to get the drivers for the RAID controller directly from Intel or LSI that will work with 2008R2 or 2012.
A third option is just to disable the RAID controller and use the disks as standard volumes, maybe replace one of the SSDs with a 1 or 2 TB hybrid drive, install Windows and other software on the SSD, use the spinning disk for data storage. As was already stated, due to the slow SATA link speeds supported on this system, you might not notice much of a speed increase with the SSDs, and a hybrid (or just a fast spinning disk) would give you more storage for your money.