The editors for the 1920 LDS edition changed the preposition here from to to in. The critical text will follow the original preposition to. The same change was made in 1 Nephi 17:14; see that passage for a complete discussion of this change. Here in Mosiah 10:15, the original past participial form arriven was first emended to the standard arrived in the 1840 edition. For additional discussion regarding arriven, see under 1 Nephi 17:14 as well as under past participle in volume 3.
Summary: In accord with the earliest text, restore in Mosiah 10:15 the preposition to and the past participial form arriven (“they had arriven to the promised land”).