You need a "basin wrench" to properly reach the supply line nut. You can get one for cheap at HD or Lowes or ? While you are there, pick up a tub of plumber's putty which might cost $4.00
Plumbing is about the "feel" of the tightening. DO NOT get all gorilla with the fittings. You should be able to get a leak-free installation with a firm tightening. Over-tightening strip or cracks things. No good.
Lets start the faucet install from the beginning because you are getting water down the lever actuator and the faucet/supply line. So, disconnect everything and start over. Pull the faucet out, clean the sink, apply a thin rope of putty on the perimeter of the faucet. Now re-install the faucet nuts from underneath using you handy-dandy new tool while keeping the faucet properly centered. (A lovely assistant can be of great help here as you lay on the floor -- but focus on your task!)
BOTH the faucet nut and the supply line need to be tightened if you look closely. Sometimes (rarely) there is a rubber washer which goes on before the faucet nut. Once you tighten both firmly, turn the water on and check for leaks. The supply line should not leak at the fitting. Get in there with a light and your fingers to be sure.
That should do the trick and your lovely assistant will think you are a genius.
PS Here is a video to assist you .... the guy is boring as hell but gets the right things done. Notice how he uses the "feel" of the basin wrench
[ame]https://www.youtube.com/watch?v=L0iyJJISqZw[/ame]