The land of Zarahemla was down from the land Nephi. Up and down refer to elevation. The reader will notice in future reading that the river Sidon had its head in the wilderness dividing the land of Zarahemla from the land of Nephi; however, the direction of flow was right through the land of Zarahemla (see Alma 16:6-7, 22:27-29).
According to Joseph Allen, as a person travels from the high point of 9,800 feet elevation near the Quetzaltenango, Guatemala, lookout to about 2,300 feet elevation at the Mexico border, to say the least there is clearly a feeling of traveling downward. The elevation in the Chiapas valley reaches its low point at 1100 feet. Guatemala city (the local land of Nephi) is about 4,800 feet in elevation. The Book of Mormon is always consistent in the fact that travelers from the land of Nephi always go down to Zarahemla, and travelers from Zarahemla always go up to the land of Nephi. [Joseph Allen, Exploring the Lands of the Book of Mormon, p. 372]