Buses do not ply direct to Aït Benhaddou but can drop you off at the turn off about 7 km away from where you will have to find a lift.
It will be better to arrange a private hire of a taxi for the trip.
At daytime, grand taxis are always waiting at the crossing. A seat costs 9 Dirham to which they might add for luggage. The cost for the whole taxi will be 60 Dirham.
Car: Aït Benhaddou is 9 km off the main Tizi n Tichka road, which links Ouarzazate to Marrakech. Coming from Ouarzazate, turn to the right after about18 km from the town. It's quite a scenic drive to Aït Benhaddou. On the way into town there is a slight turn to the right, which is a popular spot for great views over the town and the Kasbah.
From Marrakesh it will be a full day trip and the best advice is to organize a private tour if you do not have much time.
Morocco Explored  offers a well priced private trip which includes stops in the High Atlas Mountains.
Once inside the Ksar you can only walk or climb. There are two main entrances into the Ksar. One can be reached by new bridge. To reach the second, it is necessary to cross the river using the stepping stones (in dry season the river is completely dry).
The town itself is the main attraction, with mud buildings climbing up the side of the hill.
The Granary at the top of the hill is impressive and well worth the effort to get up there.
The Mausoleum of Ben-Haddou is at the base of the hill on the opposite side to the town.
Climb atop the hill for breathtaking views.
Rent a bike and drive up the road to Telouet (ca. 50km). It has been paved a few years ago but traffic is still scarce.
There are several shops within the town that sell souvenirs from all over the Sahara, particularly prevalent are Models of the Ksar and wooden artifacts from the Dogon people in Mali.
There is a very friendly carpet shop in the newer part of town near the post office.
Hotel la Baraka in the newer part of town has a restaurant with a nice courtyard.
Auberge La Goulade. The first auberge along the road into Ait Benhaddou, run by a French man and his Moroccan wife. Very friendly, excellent food and good price.
Hotel Restaurant La Rose Du Sable, Kasbah Ait Benhaddou, Morocco, ☎ +212 (0) 5 24 89 00 22, . Clean, comfortable rooms. Great food.
Riad Ksar Ighnda, Douar As Falou, Ait Benhaddou, Morocco (3km past Ait Ben Haddou), ☎ +212 (0) 5 24 88 76 44, . In an authentic yet modern setting, Riad Ksar Ighnda opens its doors for you onto the Atlas Mountains with rooms in tadelakt with air conditioning.
La Fibule d'Or Ait Benhaddou (Hicham Ouazal), ait ben haddou (ait ben haddou), ☎ +212 667597476 (firstname.lastname@example.org), . 11 rooms, restaurant, room service , panoramic terrace, free parking, free internet.200 dh per/person per night in half board.
Kasbah Tebi, Ksar Aït Benhaddou, ☎ +212 (0) 6 61 94 11 53, . Authentic renovated Berber Kasbah. Magnificent view. Tranquility. Hospitable.400 DM per person per night (halfboard).
Grand Taxi back to the crossing 9D per seat, then 14D to Ouarzazate. You might also be able to catch a bus to Marrakesh (they stop if they have seats available).