In my world, the Mayan Empire expanded into the Caribbean and met the relict ground sloths of the Caribbean. In this same world, the Mayan empire saw the use of these creatures as useful domestic animals that they could bring to the mainland. With pack animals to be used as beasts of burden, the Mayans could build even more amazing structures, and form an even more powerful civilization. They may not even have been dominated by the Spanish. Sadly, there is a problem with this idea. Could the Ground Sloths have been made in to viable pack animals?
Based on assessments of them being slower-paced in motion, having osteoderms, inturned claws on the front paws capable of tearing apart large branches, no incisors, and exclusively vegetarian diet in coproliths, I'd say it'd be possible, but probably no picnic to domesticate such a beast.
We cannot, of course, assess intelligence / intransigence, herd / individuated behaviour easily, so it's equally possible that they might be characterlogically unsuited to domestication; I'd guess though that if the people in question had a significant observational base of the behaviours of this mega-critter, it'd probably be do-able.
Sloths are not very social but they are still mammals
Most domesticated animals come from species that form packs in the wild, with cats as a notorious exception. Social species are able to live together in a confined space more easily than solitary species (try to lock two adult tigers in a barn and you'll get an idea). Moreover, their social brains allow them to "empathize" with human moods and feelings (e.g. dogs, camels, horses), and make them less likely to resist or kill their masters whenever they have a chance (google tiger accident circus to get a taste of this stuff... jaguars are worse, no circus dares to tame such beasts).
Unfortunately, it seems sloths are not very social animals. This is an excerpt from a paper on social behaviour between sloth mothers and their young offspring.
Social interactions among sloths are considered to be rare, mainly because these animals are known for their solitary habits. However, some reports represent attempts to understand to a greater extent some of the sloths' social interactions in captivity or in the wild. In this context, a study focused on indirect contact through vocalization between mother and young of Choloepus hoffmanni and Bradypus infuscatus (= Bradypus variegatus) (Montgomery & Sunquist, 1974). It showed that vocalization is quite intense and important to communication in the first 6 months of total infant dependence.
However, since sloths are mammals, they do depend on mom and learn a lot of things from her. If your Mayans find a way to substitute sloth mothers and make them addicted to humans somehow, you could have some type of sloth domestication but very different from that of horses or dogs. Maybe you could look into the process of cow domestication to get some inspiration.
I am going to have to suggest that no, sloths would not make good pack animals.
I don't say this for any particular behavior reasons. As others have mentioned sloths are not particularly social, but this I think could be trained into sloth behavior if they were raised from birth by humans. We are pretty dang good at training animals.
My concern is physiological.
Sloths move slow. Really slow. While pack animals are generally not known for amazing speed, they do tend to be able to plod along at an at least human walking pace.
Their limbs are adapted for hanging and grasping, not for supporting their weight. Their bones, muscles and tendons are not meant for crossing overland. If you haven't clicked the link link in #1, watch it, it basically tries to scuttle along on its stomach. Their claws also curve back toward the forearm, they may be able to bend them the other direction, not sure, but they wouldn't be suited to walking on dirt paths and dragging a cart or sled behind them.
Their metabolism is really slow, they can also hibernate, or enter a topor (see the wikipedia link).
All in all, nothing in their physiology suggests they would make a good pack animal.
I suppose it is possible that you could selectively breed sloths to make them better pack animals...but it would take many many generations...you are essentially trying to make them... not sloths. Considering pre-colonial peoples had already domesticated llamas as pack animals it doesn't really make sense to basically completely change the physiology of a creature.
On a cool side note the wikipedia page says a sloth can hold its breath underwater for up to 40 minutes due to its slow metabolism...which is neat.