I just installed a new sink in our bathroom. I also installed a new moen facuet and drain. After installing the piece that goes in the bottom of the sink and putting plumbers putty under it I have a water leak coming from around the big rubber gasket which fits up against the sink. I have tightened the large nut with a wrench and the gasket is still leaking. Do I need to put any plumers grease on the rubber gasket. If I don't have the plumers putty on the top side of the sink done right can that cause it to leak? Any suggestions on what to do next would be great.

Yip. Loosen the nut/washer that is located just below the rubber gasket.

Go to the top side of sink and make sure you have plumbers putty applied well. I roll up at least 1/2 inch and make sure it completely circles the area. Then fit the drain flange down exactly how it should fit. (when you tighten it, you will see some oozing out the sides - remove that later).

Now go down and make sure your pipe asembly is straight down and that you are not trying to angle it at all. Then secure that nut under the rubber gasket. If you don't tighten it well on the threads, you will have a problem getting a tight fit under that gasket. AGain, make sure everything is straight.