Natural Earth is the public domain and available for freethere are no licensing fees or royalty payments. Use it any way you like, including commercially.
Giving credit for using Natural Earth is unnecessary. However, if you wish to cite Natural Earth, use the following: Tom Patterson, US National Park Service
The author (Tom Patterson) and US National Park Service provide Natural Earth as a public service and are not responsible for how it is used and any problems relating to data accuracy.