I used these instructions to get Plesk to recognize SSIs:
1) Go to 'Control Panel' -> 'Add/Remove programs' -> 'Add/Remove Windows Components'. When the installation wizard starts go to 'Application Server' -> 'IIS' -> 'World Wide Web service' and deselect the 'Server Side Includes' option.
2) Once the SSI extensions are removed restart all Plesk services and 'IIS Admin Service' with all dependent services through the Microsoft Services Management Console.
3) Return to ‘Add/Remove Windows Components’ wizard -> Application Server -> Internet Information Services -> World Wide Web service and check the 'Server Side Includes' option and install it on the server.
4) Make sure that 'Server Side Includes' has 'Allow' status in 'IIS management console' -> 'Web Services extensions'.
5) Restart Plesk and IIS services as in step 2.
6) Log off (if you were logged on) and log on to the control panel (this is necessary for updating the component information for your session).