Cómo hacer que MAMP (o WAMP para Mac) reconozca la extensión .shtml y que entienda los includes del lado del servidor: 

http://jeffreybarke.net/2010/04/mamp-shtml-and-server-side-includes/

1. Stop the MAMP servers.
2. Open MAMP’s httpd.conf (located at/Applications/MAMP/conf/apache/httpd.conf) in a plain text editor.
3. Find the line:

   DirectoryIndex index.html index.php

   and change it to:

   DirectoryIndex index.shtml index.html index.php

4. Uncomment the following two lines:

   AddType text/html .shtml
AddOutputFilter INCLUDES .shtml

5. Find the lines:

   <Directory />
Options Indexes FollowSymLinks
AllowOverride All
</Directory>

   and change to:

   <Directory />
Options Indexes FollowSymLinks
Options +Includes
AllowOverride All
</Directory>

6. Restart the MAMP servers.