I assume you mean: incompatible to use as an SSD cache for your HDD volume? Is the SSD appearing as a drive letter now?
You're correct that the BIOS generally need to be configured to have the drives as RAID drives, not IDE/compatibility mode or AHCI.
If RAID is set, then you need to enter the Intel RAID OROM and configure those 2 drives to be in a RAID. It might even specifically list Acceleration Drive. On a "regular" non-EFI motherboard (Asus, Gigabyte, MSI, etc) you usually access the Intel OROM by hitting Control-I (that's an 'i') after POST. On the EFI boards I've seen, the RAID menu is integrated into the EFI/BIOS. On a Dell, YMMV.
Oh, another comment:
The abbreviations get a bit confusing, but:
RST = Rapid Storage Technology, which is Intel's SATA/RAID driver stack and management software.
SRT = Smart Response Technology, which is a feature included with some chipsets starting with Z68. It is managed on the OS side from within the RST program.
Also, not that it will have any effect on your current situation, I'd recommend updating your RST to the latest version.
LINK: https://downloadcenter.intel.com/SearchResult.aspx?lang=eng&ProdId=2101