Couple of things-
Install the battery correctly, positive to ground.
One of the small solenoid posts should be wired to the starter switch which should provide a ground when pressed. Be sure the tractor is in neutral.
The other small solenoid post should be wired to the side of the terminal block that has -6V when the key switch is ON along with the "-" side of the coil and also the wire to the fuel gauge thru a fuse.
It does not matter which large post is which, but one should have a large wire directly to Batt "-" and a small wire that goes to the voltage regulator "B" terminal. The other large post should have a large wire going only to the starter.
Set up this way it should start and run, if not then post back and let us know what you find. If it does run, I suggest you check to be sure the battery is charging properly with a voltmeter. -Will