It is prohibited to park on the old car park at the gravel plot (Lilla flatenbadet).
Park at the large car park at the entrance to the bathing place instead (Flatenvägen 200).
Nynäsvägen (road 73) south from Stockholm. Turn off towards Tyresö (road 229). After about 1 km you turn off towards Orhem / Skarpnäck. Turn left onto Flatenvägen. Follow the road 3 km and park at the big parking at Flatenbadet. Pass the entrance to the bathing area and follow the walkway to the right along the water. After about 900 meters you will see the crag beside the walkway.
Public transportation: Bus from Gullmarsplan towards Tyresö strand. Get off at Ältasjön. It's also possible to take the subway to Skarpnäck and walk from there (20 min walk).