This activator could be defined only in bound with a region created using plugin WorldGuard. To create activator of this type you need to use command:
/react add region <ActivatorId> [<World>.]<RegionName>
Region-activator is activating when player enters region and after every 3 second if he is still inside the region.
ReActions uses a format "
[<World>.]<RegionName>" to determine region. You can skip the world name if you region located at your server main world.