You have to use SMCLI. If you installed MDSM on a Windows system, you open a dos-window and go to c:\program files\dell\md storage manager\client (default location). From here do this command:   smcli -n MD3000i-name -c "show virtualdisk [\"virtualdiskname\"] actionprogress"; It may take a few seconds, but it should come back to tell you how far along it is.
MD3000i.  Had a 6 disk Raid 6 array and added a 7th. I'm using the Modular Disk Storage Manager Client - can't believe they didn't include a "% complete" function or am I missing something?  
Added a disk to a Raid 6 array and it is rebuilding - is there anyway to know % 
complete? It's been 16 hours...