Final answer:
Antonin Dvorak served as the director of the National Conservatory of Music in NYC and promoted the inclusion of African and Native American elements in American compositions.
Step-by-step explanation:
The individual who came to America to serve as the director of the National Conservatory of Music in New York City and who urged American composers to incorporate native materials into their works, including those of African and Native Americans, was Antonin Dvorak. His encouragement led to a greater appreciation and inclusion of diverse cultural influences in American music.