It shouldn't need formatting. That should have been done to make the RAID. If you need to erase the RAID:
1. Open Disk Utility in your Utilities folder.
2. After DU loads select the RAID volume you wish to format from the left side list. Click on the Erase tab in the DU main window.
3. Set the format type to Mac OS Extended (Journaled.) Click on the Erase button and wait until the process has completed.
You will find three entries. Two will be ghosted. Those are the disks that are combined for the RAID. A third entry would be the RAID volume and would not be ghosted. If there is no third entry then it's possible the RAID has not been made.
Check the partition information for each drive to be sure it is for OS X. If it isn't, then each disk needs to be both partitioned and formatted to use with OS X. Then you create a RAID volume using the RAID tab in the Disk Utility toolbar.