In typesetting this passage, the 1830 compositor decided that the word also should go with the preceding clause (“and the Lord said unto me also”), not with the following clause (“also I will raise up … ”). He placed a comma after the also (which was replaced by a colon in the LDS text beginning with the 1920 edition). Just before, in verse 16, we have a reference to what the Lord has already promised:
So it is not surprising that verse 18 has “and the Lord said unto me also”. Nonetheless, there is the possibility that one could interpret the also as belonging to the following clause (namely, “also I will raise up one unto the fruit of thy loins”). In verse 17, we have the Lord’s word to Joseph of Egypt that “I will raise up a Moses”. So at the beginning of verse 18, the Lord says that “also I will raise up one unto the fruit of thy loins”—in other words, there will be a second seer, one for the descendants of Joseph.
In most cases, we can find clear evidence for placing also either at the end of the preceding clause or at the beginning of the following clause. For instance, as a result of a following subordinate conjunction that, the also must be interpreted as belonging to the preceding verb say:
On the other hand, there are numerous examples of also preceded by the conjunction and, thus showing that a clause can begin with an also right before the subject. Here I list all the examples where the subject of the clause is a pronoun:
It turns out that there is only one clear case where such a clause doesn’t have the connective and,and this example is in a quotation from the King James Bible:
Besides here in 2 Nephi 3:18, there are two other cases of possible ambiguity:
For these two cases, the context favors assigning the also to the end of the preceding clause rather than at the beginning of the following clause. (For discussion, see Alma 9:4 and Alma 47:34.)
This systematicity suggests that in the Book of Mormon text proper (that is, excluding biblical quotations) the adverbial also does not begin clauses. With respect to the case here in 2 Nephi 3:18, we should note that when also follows an unto prepositional phrase at the end of a clause, the also is always attached to that clause, not to the following one:
Note, in particular, that Alma 17:11 is virtually identical to 2 Nephi 3:18 with respect to the initial clause: “and the Lord said unto also”. Nor is the Lord telling the sons of Mosiah (in Alma 17:11) to “also go forth among the Lamanites”, as if they had already been preaching among some other people. These few examples provide additional support for the decision of the 1830 typesetter to assign also to the end of the preceding clause in 2 Nephi 3:18.
Summary: Although the context allows the word also to be assigned to the following clause in 2 Nephi 3:18 (“also I will raise up one unto the fruit of thy loins”), usage elsewhere supports assigning the also to the end of the preceding clause (“and the Lord said unto me also”); all the printed editions have correctly placed the also at the end of the preceding clause.