I purchased the standalone version of RMS Mix but can't find the Sinfonia program for my editing time.
While the RMS Mix icon lives on your desktop, Sinfonia (which will unlock automatically when you unlock RMS Mix) can be found in the following places:
Windows: Program Files (x86)/RMS Sinfonia (here's a video)
Mac: Applications/RMS Sinfonia
Note: Edits must be performed on the same computer used to request the RMS Mix unlock code.