Going from memory here Rich, there's a small piece bent up out of the sheet metal floor ~ about 1/2-3/4 inch long ~ that has a small hole in it. One end of the spring goes into this.
The other end of the spring goes around the nub molded into the cam piece that sticks out below the car. If you look on that yellow cam activator piece, you'll see the piece at the end of it, closest to the sheet metal frame of the car.
Also worth noting since you bought the car used, that the yellow arm that holds the figure that sticks up out of the roof has a circular shape at the opposite end of the arm. This arm should have a metal weight inside it. Without the weight, even if everything is correctly assembled, the car will not operate correctly.
You will want the Lionel parts spring... it is made to the correct length for activation of this car. And you might have to move the dinosaur inside the car... it's potentially possible that it could interfere with the operation of the cam.
And just another note, if you didn't get the tell tale with the activation bar attached to it, you can make these activation bars yourself... and you can custom make the length to extend operation of the car. There have been some other threads on this. Musicalcraft (David from Dearborn) makes his out of plexiglass. I make mine out of the sheets of colored foam you can find at Hobby Lobby or stores of that kind.
I also like how David paints the yellow cam arm beneath the car black, which I agree, does look much better. I haven't gotten to that one yet, though I have masked off and painted the ends and roofs of these cars differing colors, which livens up their appearance as well as adding a metal brakewheel over the molded in one on the car body.