According to Mercruiser, "All Mercruiser MPI (multi-port fuel-injected engines) sterndrive, inboard, and tow sports models with serial number 0M300000 and higher are compatible with SmartCraft instrumentation systems."
It goes on to say though that to push this data to a NMEA2000 network, "The MercMonitor, levels 1, 2, or 3, can be configured as a gateway to export engine and vessel data to an NMEA 2000-compliant chartplotter."
So, from that, and some Google-Fu, I was able to deduce that without the Smartcraft package, your system doesn't spit out NMEA2000 sentences and does not have the hardware to even interface with NMEA2000. What I'd do (and did when I installed my HDS-9 GenII Touch) is purchase one of these from NoLan Engineering
http://nolandeng.com/rs11.php. It does a fine job of taking the analog data from your gauges and convert them to NMEA2000 sentences for use on the NMEA2000 network. Be sure to add a NMEA2000 fuel flow sensor and an exhaust temp sensor and/or cooling water flow sensor (either can interface with the NoLand RS11) into the package to finish off the data acquisition and alarm capabilities. It was interesting to find out that our 268 gets around 2MPG while on plane at cruise (~30MPH) or at a slow putter above idle. Right on the "knee", however, she certainly sucks the fuel if held there. Basically MPG decreases from about 2.5 at idle in gear all the way to ~1 right at the knee, then as the bow drops it goes back to 2 all the way through just over cruise where it again starts it's drop towards 1MPG at WOT. Much better than I would have thought from a boat this size. Having a configurable alarm for all the engine operating parameters is a wonderful piece-of-mind feeling.