If you prefer to visit Chambok on your own, then follow these instructions:
From Phnom Penh, take National Road No. 4 towards Sihanoukville. After 88 km (2-2.5 hours) traveling on the highway, you will arrive in Treng Trayeung town. Follow the signs to Kirirom National Park and turn right at the main intersection. After about 10 km you will see the entrance of Kirirom National park. Here you take the gravel road to your right and follow the signs for Chambok. It is another 10 km before you see Chambok Pagoda; the entrance to the ecotourism site is on the left, just before the pagoda. See a map here.

You can either hire your own vehicle, take a share taxi or hop on one of the many buses leaving for Sihanoukville from the Central Market (Psar Tmai). From the turn-off at Treng Trayeung you can find a motodup to take you to Chambok village.