The OpenStage SDK is located within the Windows install directory for the OpenStage Client on you Client Computer.
There is a Windows help file located within the Client Software's Installed Directory. Within that help file there is a list of all available SDK files as well as basic directions for use:
...\OpenStage Client X.X.X.XXXXX\help
typical install: C:\Program Files\Organic Motion\OpenStage Client 220.127.116.1183\help
The actual SDK files are located:
...\OpenStage Client X.X.X.XXXXX\include\om
typical install: C:\Program Files\Organic Motion\OpenStage Client 18.104.22.16883\include\om
Within this directory you will find all SDK files within the "sdk2" sub-directory along with some ancillary utilities in their respective sub-directories.