First of all to see debug statements inside DebugView you need to compile your assemblies in "Debug" mode. For optimization reasons if you compile your assemblies in "Release" mode all the System.Diagnostics.Debug.WriteLine statements will be ignored by the compiler and you won't see any output in the DebugView.
When it comes to BizTalk project sometimes even though the tool bar will say the build is "Debug" as shown in the figure
When you actually look into the properties of the BizTalk project it may be configured for "Deployment", which is equivalent to the "Release" build and it won't emit any debug symbols.
So, in this case you need to set the configuration to "Development", which is equivalent to "Debug" build.
Nandri!
Saravana