During the installation of Silk Central if you select Custom Installation in order to install Silk Central over IIS you may not receive IIS as an option as shown below:
The reason for this is that the service "World Wide Web Publishing Service" (short name: W3SVC) is not running when the installation of Silk Central was started.
To resolve start this service and begin the installation of Silk Central again. Once this is done the "ISAPI Web Server" should be available during installation.
For more details on IIS configuration please see the Silk Central Installation Guide.