Actually, I'd say it does look like you have SSI support. Without SSI, the command would've simply been read as a comment and silently ignored.

Without any details on the error you saw, I'd guess that it's complaining about not being able to find "/common/head.inc", right? What I wrote was just an example (I could have been more clear) but the real point is that you should probably next go to one of the references you've received here and try some real SSI directives (including files that actually exist).