Jmartincic,  You can simply reseat the failed drive to initiate a rebuild. In the OMSA gui with ESXi it is only a monitoring tool. You can't initiate anything from it though in regards to the controller.   
I'm an owner of a Dell PowerEdge T420 with ESXi 5.1 installed and I've been wondering whether it is possible to rebuild a RAID array without rebooting the server. I was under the assumption that using Dell OpenManage running on Windows Server  in conjunction with the Dell OMSA VIB would allow management of Virtual Disks while the system is online.   I am able to see the health status of the RAID arrays through OpenManage, but cannot seem to perform any management of the arrays. Was I mistaken in this assumption?  
Dell OpenManage, ESXi 5.1 & rebuilding RAID arrays online