Here the 1874 RLDS edition accidentally omitted the preposition of from the phrase “the land of Zarahemla”. As explained under Alma 2:15, the original text of the Book of Mormon has examples only with the of for this phrase. Here in Mormon 1:6, the subsequent RLDS edition (1892) restored the correct phraseology with the of.
Summary: Maintain the of in the phrase “the land of Zarahemla” everywhere in the text, including here in Mormon 1:6.