At the beginning of the verse, the Lord is still speaking, and repeating the call to Israel as in verses 12 and 14. At this point, the Book of Mormon text diverges slightly from the Isaiah text as we have received it.
Ludlow's citation of the New King James Version is "Draw near to Me [and hear this]; From the beginning, I did not speak in secret; From the time anything was declared [existed], I have spoken [was there]. (Ludlow, Isaiah, Prophet, Seer, and Poet, p. 405)
Gileadi renders it: "Come near me and hear this: I have not made predictions in secret; at their coming to pass, I have been present. Now my Lord the Lord has sent me; his Spirit is in me." (Gileadi, The Book of Isaiah p. 192).
The concept of the call to Israel and the declaration that the acts of the Lord have not been is secret are clearly stated. The question comes in understanding the import of the references to the past. Whatever the translation selected, the clear intent of the Lord is to emphasize that He does not work in secret by indicating that He has declared his word and intentions from the beginning. Thus the Lord is reminding Israel that He is consistent in declaring his intent (through prophets - the unstated but assumed reference given the surrounding text).
The final phrase of the verse is interesting in that it once again appears to present a shift in locutor. The easiest explanation is that Isaiah has fallen out of the first person citation of the Lord, and added his personal note to this passage. This might indicate that we are to see Isaiah as one of the fulfillments of the prophetic statements about an individual.
A more radical interpretation, possible in English, but personally unknown for the original text would be that this phrase becomes an early presentation of the triune Godhead. The history of the understanding of the Isaiah passages would argue against this speculation - fun though it might be.