wwat is incorrect. Revelation is the last book in the Bible, but not the last book written.

First of all the basis of the critics' argument is flawed. The word "Bible" means a collection of books, not one book.
John is not talking about the entire cannon of books of scriptures we call the Bible, he is only talking about the words and prophesies of "this book" (his book) that he wrote. The plagues he is talking about are those found in the same Book of Revelation.(See Rev. 9:20, 11:6, 15:1-6).
A similar scripture appears in Deut. 4:2, where Moses says:

"Ye shall not add unto the word I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the Lord your God which I command you."

Does this mean that everything written after this scripture is worthless or wrong? Also, some of the New Testamant books were written after Revelation. In fact, the entire cannon of Bible scriptures was not completely complied, as we have it now, at the time John wrote his scripture. The earliest known listing of New Testament books is the Muratorian Cannon, ca AD 160-170 which omits 2 Peter and 3 John. It was not until 365 AD that all 27 books as we have them now were officially listed in the New Testament by Arthanasius, Bishop of Alexandria.

It is also interesting to note that John himself added to scripture after writing the book of Revelation, which is generally conceded to have been written while he was on the Isle of Patmos.

It was long after John left Patmos that he wrote his first epistle. This fact alone would be sufficient to defeat the claim that revelation was closed and that no more scripture was to come forth.
The LDS Church has never added anything to John's Book of Revelation; they use the Kings James version as it is just like most other Christians.