zMAX, or any additive, is most likely not the cure for what ills the Honda. There are thousands of reasons a car shakes...ignition miss, valve stuck/burnt, a tire out of balance. The idling lower and shaking engine symptom sounds like an engine problem...maybe a sensor gone bad, spark plug, plug wire, vacuum leak...none of which can be fixed by anything other than repair to the problem. The shake on the highway could be worse, as in dangerous, if it's attributted to a tire/suspension problem. Attack the 'engine idle/shake/rough' problem first. If you're lucky, that may take care of the highway shake. Don't ignore the highway shake problem, though.
zMAX is not, and I don't care what the commercial says, a cure for a real engine problem. If it's carbon around the valves, yeah, maybe. Fuel injectors dirty, OK, maybe. It will only help an engine perform better if the engine's already in good working order. Nothing you pour into the crankcase is going to fix a mechanical or electrical problem. I think zMAX would like for everyone who has a real engine problem to buy the stuff, then finally fix it later, after being frustrated. Most people who try this approach will simply forget the $39.95 spent on zMAX and just be glad it's fixed. Don't do it people!