If you can't get SSI working on Mac OS X, try looking for a special user.config file which contains settings for sites in user folders.
I spent all morning trying to enable SSI to no avail. I was convinced my edits to the httpd.conf file weren't working. When I searched my hard drive for the string "httpd" (using unix command 'locate' since Finder's search won't see folders like "private") I came across some .conf files prepended with various usernames. I found one for myself (zimboden), so I opened it and found inside:
Options Indexes MultiViews
Allow from all
This looked awfully similar to the entries that I was editing in the httpd.conf file, so on a lark I changed the Options line to read:
Options Indexes MultiViews +Includes
and VOILA! finally SSI started working for me!