Have you run the setup wizard? If so, you could've done it from the wizard, but it doesn't matter eitehr way.
Browse into the control panel from your computer. Click on the far rigth hand icon to Login. One logged in, go to the Storage section. You shoul see a panel reporting the existance of your two disks.
Go into the RAID section. Click on your choice of raid type - Generally RAID1 for the safety or RAID0 for speed and capacity but no redundancy.
Leave the other settings the same. And click on "Create".
THIS TAKES about four hours - depending on disk size.
ONCE this has finished, you may need to load new firmware. If you keep getting a box saying "complete the FW upload" and it won't let you do anything else, go into the System tab, click Firmware, use the Browse button to explore the CD and find the firmware upgrade that came with the machine, and Upload it. This takes about five mins and once it's complete you MUST reboot the unit, which takes two or three more minutes.
Then you're done. Create folders and users, turn on services, it's ready to go.