The RAID BIOS typically is entered after POST. You do not need to enter the computer BIOS.
However, in order for the RAID controller to activate, you do have to enable RAID mode in the computer BIOS. Since you're getting in there already, make sure that you have RAID mode enabled for your SATA devices, instead of AHCI or IDE mode (AHCI is typically the default).
After you set it to RAID, exit the BIOS, and once you reboot and the computer completes POST, before the OS loading step, you'll see a screen where it initializes the RAID controller. It will typically then display a message stating "Press Ctrl+I to enter RAID setup..." or a similar message.
Keep in mind that if you have anything on your hard disk before switching to RAID mode, you will have to back it up. Changing to RAID will require the disk to be reformatted, and for you to reinstall the operating system. This is because the OS installs with drivers that tell it how to communicate with the disk. There are separate drivers for IDE mode and RAID mode. If you change modes, it can't use the IDE drivers and will fail to load.
Unless you want to try using a backup/restore system like Acronis TrueImage's Universal Restore feature, a complete operating system reinstall is required. During the installation step you will have to point to the RAID drivers on a USB drive or CD or something, so make sure you have them handy.
NOTE Since you do have an Intel setup, it is likely that you will not have to reinstall the operating system, and that the Intel Rapid Storage Utility will let you set the RAID arrays up, even within the OS for you without losing data. Of course, there's always a risk to lost data, regardless, so make sure you have your data backed up. But this should make the process a bit more easy.