Jaccuzzi Perfecta EZ33959 Toilet

I bought this toilet at Lowes and when I installed it I found that water leaks from the tank. If I shut off the water supply it leaks completely. Water does not go onto the floor so I know it is getting past the flapper assembly. I used joint compound on the big nut underneath and on the big rubber gasket and tightened the nut with some large channel locks. It still leaks and the only thing I can think of is to take the tank back off and put gasket sealer under the flapper assembly on the inside of the tank.
Does anybody have any ideas on a solution?

Thanks,
Larry

Are you sure it's not not flapper?

I've never used joint compond on a toilet before.

These are our instructions
They should work for any brand.