First thing first, you have to includes make sure APache knows which file to be parsed with should be parsed using SSI
At /etc/httpd/conf/httpd.conf, ensure the following has been done.
AddType text/html .shtml AddHandler server-parsed .shtml
The next thing is to ensure that the Directory Section of the Apache where the .shtml reside should contains this. This is epsecially true if you set the AllowOverride
<Directtory /home/tester/public_html> Options +Includes AllowOverride All Order allow,deny Allow from all </Directory >
For more information and resources, see