1. By Bus:
Take a Phuong Trang sleapping bus from Ho Chi Minh City to Da lat, on the way to Dalat, you ask the driver that you wanted to get off at the junction for Cat Tien National Park which is clearly signposted. After dropped off the bus then you shold take a motorbike drivers, who will drive you to the park wih a distance of 28kms.
2. Provate car:
Book a private car or taxi from Ho Chi Minh City with many car rental company, it was easier for you because you do not need to dropp off along the trip. But it ismore expensive than the slipping bus.
I hope this has been helpful. Good luck!