The controls for GrassZone are really quite simple.
On mobile devices you can move around with the on-screen keypad. To see your key count or get off of a cart you can press X. To sprint or get onto a cart, you can press O.
On a desktop or PC you can move around using arrow keys To see your key count or get off of a cart you can press x. To sprint or get onto a cart, you can press z.
On a controller you can move around with the key buttons. To see your key count or get off of a cart you can press X. To sprint or get onto a cart, you can press O.
The golden is located in a very wide room to the bottom right of the bridge. All you need to do to collect it is to simply step on it.