In apache, find the httpd.conf, and find this line:
Add index.shtml to the end of that line.

mddv are you using Apache or IIS or something else for the webserver?

Exactly, and if I remember correctly apache check them in the order they are written as well. So if you use for example both index.shtlm and index.html you can decide which one to look for first. Not that I can think of a situation when you do, but there must be, since the option to is there. Or maybe not.

You should be able to write an .htaccess file and have it direct to your .shtml file instead.

so if I wanted both would it be:
DirectoryIndex index.html index.shtml ?

There should be a long lisrt in httpd.conf, and it is displayed in the order it is listed in that file. Here is a typical file:

DirectoryIndex index.html index.wml index.cgi index.shtml index.jsp index.js index.jp index.php4 index.php3 index.php index.phtml index.htm default.htm default.html home.htm

So if there are both index.html and index.cgi in the same directory - index.html will be displayed when no file is specifically called in the URL.