I have a river network within a watershed of just under 5000 square km. The digitized direction represents the direction of flow. Apparently there are two ways to set the flow direction: 1) Identify sinks and sources. I don't have either, and can't think of a way to quickly generate them for over 40,000 segments. 2) Use a tool such as this: http://edndoc.esri.com/arcobjects/8.3/Samples/Network/Utility%20Network%20Analysis/Flow%20Direction%.... I have tried installing the .dll but I get a message saying "no new objects were added". Could this be because I don't have administrative rights on this machine?
FYI I've got ArcGIS 10 and was able to add the DLL to ArcMap without any error and then drag the button onto a toolbar although I did not use it. This would indicate that it's a permission issue for you and you need to get IT "support" to help you.