See http://www.freepbx.org/trac/ticket/2441 (and add a comment there if you feel like it) but it appears you are not the only one to get this error. I get the feeling that someone contributed a not-quite-ready module in this case and then failed to offer any further support for it (a common problem for contributed modules).

If you look around in the /var/www/html/admin/modules/ directory you should find a subdirectory for this module (not sure what it's called since I've never had a need for it personally) and inside that directory there should be a functions.inc.php file which I believe is the file that's supposed to get called when you try to access that module. If the file doesn't exist, that's why you're getting the error. If it does exist, then something in the installer probably isn't writing the link properly. Another issue could be if the permissions and ownership inside the module are set incorrectly (check the other modules, then see if this module's permissions and ownership are set the same).

Also, since I'm telling you that much, I'll add that if you should happen to get it working, you should never use the capabilities provided by this module on any system unless all users that are subject to monitoring are informed in advance - or if you don't want to do that, then you'd better talk to an attorney and make sure you stay within the bounds of the law. In some jurisdictions and/or under some circumstances, the use of this module to listen in on a conversation could constitute a felony, unless both parties are notified in advance of the possibility of monitoring (there's an Asterisk sound file, "this-call-may-be-monitored-or-recorded", that can be used for notification on incoming calls). Just because it's technically possible to do something doesn't necessarily make it legal.