You need a permit to access the forest reserve and waterfalls. A local guide is recommended because it's easy to get lost in the thick forest and - by the way - you learn a lot more about the gorgeous surroundings and village culture.
Definitely get a guide for this walk - not only do they know their way around the forest, but they can also tell you about the trees, animals and local culture. Try Usambara Eco Tours. Waterfall is small but very pretty.