Dealing with Rev. 22:18-19 itself is quite easy, yet it continues to be cited as if it were a legitimate slam-dunk against the Book of Mormon. The prohibition against adding or subtracting from the word of God actually goes all the back to the time of Moses, who wrote the following in Deuteronomy 4:2:
Ye shall not add unto the word which I command you, neither shall ye diminish ought from it....In Rev. 22:18-19, John echoed the words of Moses as he concluded writing the Book of Revelation:
For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:Moses and John were absolutely correct: no man has authority to add or subtract from the word of God. But Deut. 4:2 did not keep Moses from writing additional chapters, nor did it prohibit Isaiah, Malachi, Matthew, Mark, Paul, and even John from writing later scripture as directed by God. It did not mean that God could give no more revelation or scripture, but that the inspired words of God given to his apostles and prophets should not be altered by men.
And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
Read the text carefully of Revelation 22:18-19 and ponder what John is talking about. At the time, there was no Bible as we know it. The new Christians had the Septuagint (which included the Apocrypha) and scattered writings of some of the apostles, but there had not yet been any known attempt to establish a New Testament canon or to bring the Gospels and epistles into a single volume. John, who was in exile on the Isle of Patmos, is obviously referring to the newly written text before him when he speaks of "this book," the Book of Revelation. He refers to the unique contents of his book: its prophecies, its descriptions of plagues, its discussion of the holy city, and urges that no one change what he has written. Even though the Book of Revelation has been placed last in our Bible, it was not necessarily the last book written, but may have preceded other writings of John himself by a couple of years. In fact, many Christian canons over the centuries did not include the Book of Revelation at all, and even Martin Luther questioned its status. The first church council that listed most of the canonical books in our present Old and New Testaments, the Council of Laodicea that met in A.D. 363, still did not include the Apocalypse of Saint John [Bernstein, p.5]. The common idea that this was the last book added to an existing canon of New Testament scripture by John is erroneous, as is the idea that John meant that there could never be any more scripture.
Latter-day Saints fully agree with John: no man should change what God has spoken. However, God has the authority to speak what and when He wants. God spoke to other prophets after Moses and many of their divinely commissioned writings have been preserved in the Bible. God also speaks today to living apostles and prophets in the Church of Jesus Christ of Latter-day Saints, and we should be willing to accept those whom God has sent and hear their inspired words.
When God speaks to prophets, they write words that become scripture. Moses, Isaiah, Matthew, Luke, John, and many others all added scripture. One of the surest signs that the Church of Jesus Christ has really been restored is that new scripture has been added! The Jews at the time of Christ claimed to revere dead prophets but rejected living ones and rejected newly added scripture. They were in apostasy. Those who reject new prophets and new scripture from God in our day are likewise in apostasy and need to repent and come unto Christ more fully.
Now back to Revelation. Can you read it without seeing that God will yet do and speak many things in the future? Two prophets of the last days will prophecy in the streets of Jerusalem and be killed (Rev. 11). Angels will declare messages (Rev. 14:6 could even be a reference to the angelic ministry of Moroni and others that helped bring forth the Restoration). Christ and God will speak. Amazing works of God will take place. And saints (members of God's church) will not only work day and night in the restored Temple during the great Millennium (Rev. 7:15), but they will surely study and ponder the records of God's great dealings with man in the past. How dare we presume that God won't allow His obviously ongoing words and deeds to be recorded and studied as sacred writings?
The idea that the current Bible is the end of God's record, that the canon is closed, and that prophets can no longer speak, is a MOST UNBIBLICAL heresy. That doesn't prove that the LDS Church has authority and true revelation, but after a careful reading of Revelation, there should be no question that those who claim there can be no more revelation have Revelation completely backwards. Tragically so, IMHO.