Send that trainer back to school!! No, ellipticals can't 'build mass' (I assume that when the trainer said 'mass', she/he meant muscle). Cardio exercise - like the elliptical - isn't muscle building exercise. Only Me is right - the elliptical's fantastic exercise to make your legs tighter and toned, but if you want to build muscle in your legs, you gotta hit the leg press and squat rack.
Think about how we build muscle - we lift a heavy weight for maybe 12 reps, to the point of muscle fatigue or failure. What we're doing is stressing the muscle and making tiny microtears in the muscle. As the muscle repairs (while we're resting), it grows back stronger.
Now think about cardio - I don't know about you, but when I do the elliptical, I don't do 12 reps and hit failure.
I think it's more like 6000 strides! And any exercise that you can do for 6000 reps builds cardiovascular fitness, endurance, and burns calories -- but doesn't build muscle.
Hmmm, seeing as I do between 30 and 60 minutes of elliptical every day, I should have incredibly muscular quads, shouldn't I? Not hardly!