If you only have a single "disk" listed in Disk Management, then you only have a single VD configured, which is good. You need to install OMSA to expand your VD. You then insert your new disks, make sure they show as Ready under Physical Disks, then go to Virtual Disks and choose Reconfigure for your RAID 5. Adding two disks to your RAID 5 will add about 500GB (bit math makes it a little less) to you current Windows "disk". After the reconfigure, you will have something that looks like below. Then, assuming you aren't running 2003, you will right-click the D: drive and choose Extend to finish with the second picture: [IMAGE]
I currently have 3 250gb drives setup as a raid 5 (original configuration) on a R710 server with a Perc 6/i controller.  I have ordered 3 more drives (same drives) as we are running out of disk space. There is a C: and D: drive configured.  Can someone point me to instructions for adding these additional drives to the existing raid (D drive) without having dataloss?  Thanks Gary 
Adding additional drive space to Perc 6/i R710 Server