By default, includes only happen in pages with .shtml extension. But there are two alternatives if you don't feel like changing the extension of all your files. One involves the X-Bit Hack; use this if you only want some selected pages to use includes. The other involves an AddOutputFilter line.
I assume you've already set
The other line is
:: shuffling papers ::
AddOutputFilter INCLUDES .html
Replace ".html" with whatever extension your pages currently use. This refers to the physical file, which might be different from your visible URL. This line means that all
pages in .html will be checked for includes.
If includes aren't happening at all, you will see the
<!-- include blahblah
line in the html of the finished page, just like any other comment. If they're theoretically possible, but there was some other error, you will instead see the Include Error message. You will want to change this to something like
SSIErrorMsg "<!-- SSI error -->"
because the default error is a bit of plain text that's visible in the page if you know what to look for. Not helpful to the end user.