That's the correct markup to use (minus the slashes).
Using virtual includes for SSI means the target is going to be processed by Apache. So if your target is a script or other program, its output must start with an HTTP header section.
Also, you should be able to browse to the SSI script - have you tried browsing to
If you get an error then the problem is with the script, not your markup. Maybe you need to chmod it to 755. One thing to keep in mind is that a script called through SSI will have a different environment than when requested by a browser and that may generate a different set of errors from the script.