I have been flying down every year for the last 10 years. for arrivals Friday pm and all day saturday you will require a slot. This can be arranged by fax or email. Arrival is usually via a reporting point nrth of le mans. All very simple and relaxed as longs as you avoid late lunchtime on race day arrivals.
Departure on race day requires a pre-arranged slot which is organised when you file your flight plan. this should be done when you arrive, you will then be given a departure number. However, IFR heavy traffic take priority and I have waited 45minutes for permission to start engines!
The taxi situation is dire from the airport and advance booking would be a great idea. We usually head for the bar and just relax!!
This year they will have the tram system running from the Town to the circuit this will make access much easier. good luck