At Nice, you go through security and you are in the Schengen area. You then go to the right and pass through passport control to the non-Schengen gates.

The lounge you want is in the non-Schengen area, so it is after passport control. Most likely your gate will be very close.

There is also a lounge in the Schengen area. If you try to go into this one you will be directed to the other.