적 is a noun
1. a point of time, when
그는 어릴 적에 시골에서 자랐다
He grew up in the countryside when he was a child.
서울에는 한 번 가 본 적이 있다
I've been to Seoul once
여섯 살 means "six years old"
여섯 살적 means "When I was six years old"
~한 적이 있다 means "have experience in something"
본 적이 있다 means "I have experience in seeing" or "I have seen"
제목의 means "with a title". It is same to 제목을 가진
체험하다 means "expericence"
체험한 means "which is experienced"
체험한 이야기 means "story which is experienced" so "True Story"
여섯 살적에 나는 《체험한 이야기》라는 제목의, 원시림에 관한 책에서 기막힌 그림 하나를 본 적이 있다 means
When I was six years old, I have seen a magnificent picture in a book about the primeval forest with a title of "True story".
Following is the translation of same sentence:
In Enlish book,
Once when I was six years old I saw a magnificent picture in a book, called True Stories from Nature, about the primeval forest.
In spanish book,
Cuando yo tenia seis anos vi en un libro sobre la selva virgen que se titulaba "Historias vividas", una magnifica lamina.