I have a group of 3 and to get them to breed I put them through a "dry" season and then a "wet" season. To accomplish this I limit my misting to about once a week, sometimes not even that often, this will allow the tank to dry out some. I do this for about 6-8 weeks. Then I will gradually mist more often and heavier at least 2 or 3 times a day. It is also important to note that you should also increase feedings at this time. If all goes well you should start to see breeding behaviour in about a week or two. Eggs should follow shortly, I recommend keeping moss and a decent piece of bark in there also, it has been my experience that they prefer to lay eggs under these items. These mantellas are very susceptible to heat stress, so I would also keep that in mind, anything above 79'F (15'C) will be deadly to them.
Hope this will help,
Justin