The bureau organises documentation for visits to Nuer villages. A local guide (Birr500 per group) is compulsory, as is a letter granting permission to enter a village (Birr150). These will take about 15 minutes to sort out. They also offer general tourist information, as well as advice on visiting nearby villages and Gambela National Park.
Note that the bureau is closed at weekends and on holidays; time your stay to coincide with weekdays. If you do arrive at the weekend try calling Aychlum, who works at the tourist office and will hopefully come and sort everything out for you.