SSI are by default turned off for performance reasons. To turn on SSI, please follow this guide:
In the directory where SSI is required, create a new file named ".htaccess".
The ".htaccess" file will contain the line:
The page where you wish to have SSI function should end with .shtml instead of html so that the webserver will process the SSI. For example, if your page is named "mypage.html", you have to rename it to "mypage.shtml"
In the page where SSI is required, place the line <--#exec cgi="path to your script"--> at the appropriate location.