Genesis 5:24 And Enoch walked with God: and he was not; for God took him. Hebrews 11:5 By faith Enoch was translated, that he should not see death, and was not found, because God had translated him: For before his translation he had this testimony, that he pleased God. Jude 1:14 And Enoch also, the seuenth from Adam, prophesied of these, saying, Behold, the Lord commeth with ten thousands of his Saints, Jude 1:15 To execute iudgement vpon all, and to conuince all that are vngodly among them, of all their vngodly deeds which they haue vngodly committed, and of all their heard speaches, which vngodly sinners haue spoken against him. The quote which Enoch makes in Jude 14 is found within the early portions of The Book of Enoch... Therefore we may conclude that the disciples of Jesus The Christ studied The Book of Enoch. This is the most common explanation to how they were able to quote Enoch.
About the Author
The author who revised The Book of Enoch made as (little) and (few) revisions in it as possible. For he desired to have the original text. But the fact is that there were several portions of the scroll of Enoch which renders various verses missing. There were literally pieces of the document of Enoch which were lost or destroyed. And so the author, having already written the entire book of Nathan the prophet and also having written the book of Gad the seer, felt well qualified to fill in the missing parts of Enoch. So without tampering with the parts of Enoch that we do have, the author, by faith, filled in the parts that we don't.