OK, so you're not running OS X Server, so you can ignore my suggestion to use Server Admin.
You have this discussion running in two different threads here at this point (also in
this one). It's also hard to tell how much customization you've done to your httpd.conf file, but one option you have is to back out of your current configuration file. In the /etc/httpd directory is an "httpd.conf.default" file, yes? You could always start with a fresh copy there. ("sudo mv httpd.conf httpd.conf.customized", "sudo mv httpd.conf.default httpd.conf", "sudo apachectl restart")
There isn't much that is different about Apple's Apache installation versus a "default" installation for most purposes. I typically dislike the "have you read the manual" advice, but have you read
Apache's documentation on SSI? It gives pretty good explanations of each of the necessary steps.
If you want to enable SSI for the default on your server (typically located in "/Library/WebServer/Documents"), then go to the section in your httpd.conf file (approximately line 390) that begins with "<Directory "/Library/WebServer/Documents">". The line that looks like:
Options Indexes FollowSymLinks MultiViews
change that to
Options Indexes FollowSymLinks MultiViews +Includes
Also, make sure "LoadModule includes_module" is uncommented, as well as "AddModule mod_include.c".
Then, further down the file, in the section "<IfModule mod_mime.c">", make sure the following two lines are uncommented (mine are located around line 888):
AddType text/html .shtml
AddHandler server-parsed .shtml
Aside from that, as Roger said in the other thread, if you're still getting errors, you really need to look more closely at Apache's error log, as the browser error message is usually very uninformative.
You can use Console to read the error logs if you prefer a GUI to the command line. (Open Console, click the "Logs" icon, expand the "/var/log" item, then expand the "httpd" item and examine the contents of "error_log".
If you're still getting errors executing an SSI, post back here with the error you're getting and we'll take it from there.
Wes