Khao Lak doesn't have an airport, but it's well connected to towns and cities that do, so it's never that hard to get into this collection of villages. In fact, when you fly into Phuket or Krabi, you can find people-carrier services that take you straight from the airport to Khao Lak. Of course, taxis, buses and car hire are also available to get you from the airports to the villages.
There are no airports in Khao Lak, but the airports in Phuket and Krabi are fairly close. Both serve international destinations too, so there's a good chance you'll be flying into one of them.
Major train stations
The closest train station is in Surat Thani, which is on the east coast and a good 124 miles away. If you want to ride a train, you can take the railway from Bangkok to Surat Thani, then take a bus or taxi from there to Khao Lak.
Reaching Khao Lak by water is also an option since it's on the western coast, and several islands in the Koh Lanta region offer ferries to Khao Lak. This region is accessible from Krabi, though taking a bus to Khao Lak is technically faster.