The 1841 British edition accidentally omitted the narrative connector now in this passage; the subsequent LDS edition (1849) restored it. Either reading is, of course, possible. There are a total of 105 instances in the original text of “and now it came to pass”—and many more without the now. So in each case, we follow the earliest textual sources. For other instances where the text has omitted the now in this context, see under Alma 8:1.
Summary: Maintain the original use of now in Alma 45:15 (“and now it came to pass that …”).