Well, the place was visited by me in 1999 but I guess that most of the information is actual. The easiest way to get there is by car (I haven't even checked the possibility of local transportation there), from Tunis take road P7 to Mateur and then P11 to Tinia, after a while turn left on a non-sealed but stable road you can reach the gate of the park (there is an entrance fee for the car). Just around 2 km from the place there is an eco-museum (at the time of my visit it was free of charge) with local exhibitions. From that point you can take an easy by-the-lake walk on a local road (unsealed) round the lake. All that area together with a nearby Jabal (Mointain) Ichkeul and the marchlands with bulls (on a way to the park) are inscribed as World Heritage Site. More info:

www.eoearth.org/article/Ichkeul_National_Park,_Tunisia