Hello! The DLL is automatically registered when the sample is compiled.
You can see how it works if you open the file CutomTrafficProvider.csproj in Notepad++ or some other text editor. Scroll down to the bottom of the file and there are "BeforeClean"and "AfterBuild" sections telling the project to unregister before building and register after.
If you think it might not be working, try to register the DLL manually just to make sure it can. For ArcMap, it would be something like this:
c:\ProgramFiles (x86)\Common Files\ArcGIS\bin>esriRegAsm.exe <Path to your DLL> /p:Desktop