There is no direct way of accessing the RPD once OBIEE12C is installed.No Administration tool is available by default.
We have to download and install client tool, and using this client tool we can able to work on RPD, catalog manager, job manager.
Download the Client tool from the location
and extract the OBIEE 12C Client tool and run the setup file
Provide a path where you want to install (It is better to provide a path outside the OBIEE 12C installation folder).
Wait for the Process to complete.