The preposition of was dropped here in the 1837 edition, probably accidentally. As explained under Alma 27:22–24, for each case of “land (of ) X” we let the earliest textual sources determine whether the of is present. Here in Mormon 1:3, both 𝓟 and the 1830 edition have the of, so we restore the of in the critical text. The RLDS text made the restoration in the 1908 edition. This is the only example in the text of the name Antum.
Summary: Restore the preposition of to give “the land of Antum” in Mormon 1:3; both the 1830 edition and the printer’s manuscript have the of.