What nails or screws to use to support 600 pounds max? Making tree house bed
Common 3-1/2" gold construction screws (of good quality) are good for 50# or more each when used in solid lumber and penetration is at least 1-1/2". Dividing your bed into two ends, you'll need 6 screws per end to hit your load rating. Considering that you'll probably anchor it along the side as well, you'll have more than adequate support. 

Do not use nails for anything that's not intended to be permanent, nor for anything where pullout resistance is critical. 

Disclaimer: This assessment is based on several decades of construction and home ownership experience, and not any particular specification or product rating.