The solution is to make sure that Options Includes is turned on and that either XBitHack is turned On, or that you have the appropriate AddHandler directives set on the file type that you are using.
As discussed in Recipe 8.8, there are a number of ways to enable SSI. If the unparsed SSI directives are appearing in the HTML when the page is loaded, this is a clear indication that SSI execution is not enabled for the document in question.
If the server has difficulty parsing an SSI directive, it will substitute the phrases "An error occurred while processing this directive" in its place in the response. If this happens, the cause of the problem should be listed in the server's error log. See also Recipe 8.12.