To reach Pailn from Phnom Penh or Siem Reap by bus you will have to go to Battambang (6 to 7 Dollars) and then change bus at the Central Bus Station to go to Pailin (4$). There is also way to hook up a Taxi from Phnom Penh at the Central Market (Psa T'Mai).
There are not many buses from Battambang to Pailin, buses leave at 7.30am and 8.30am, the last one leaves at 13h00 but there is a small taxi station dedicated to commuters to Pailin and a local moto-dop or Tuk-Tuk will take you there for under a Dollar.
From Thailand, Bangkok, the North and the islands, the easiest way is to go to Chanthaburi and from the bus terminal ask a local taxi to bring you to the minivan for Baan Pagat, the Thai name for the border crossing point, called Prum on the Cambodian side (200 THB).
At the border you may have to bargain your way with a local taxi driver, from 3 to 10$ depending on how many persons and luggage, sharing taxi or taking it whole.