The manual is pdf which is automatically installed on the Client Machine with the OpenStage Client (BackStage). To access it launch the client software, BackStage, and navigate to the Help menu and select "OpenStage Manual." There is a keyboard short-cut of F1 to access this as well.
Alternatively, you may access the pdf directly by navigating to the software's installation directory -- "..\OpenStage Client XXX\bin\manual.pdf". In a typical installation this would be something like:
C:\Program Files\Organic Motion\OpenStage Client 18.104.22.16883\bin\manual.pdf
You will need to have Adobe Acrobat Reader (or compatible pdf viewer) to view the manual.