Server component firmware updates of iDRAC7 and above from the CMC, require a Dell-authorized SD card installed on the active CMC.
Once installed, navigate to Chassis Controller-> Flash Media -> Configuration and ensure that you have the feature activated.
One option is to use the free software Dell Open Manage Essentials (OME) to perform the firmware update of multiple servers https://marketing.dell.com/ome-software
Another option that requires some scripting knowledge is to use WinRM or WSMAN. Examples: http://en.community.dell.com/techcenter/extras/m/white_papers/20066174.aspx
A new feature starting in CMC 4.50 and iDRAC 1.50.50, enables the use of a network share (CIFS/NFS) to host the server firmware DUPs and perform the update from the CMC across multiple servers.