"DllRegisterServer entry point was not found" mean it is not a COM DLL.
" runtime error 53 : file not found: rscomclNoMsg.dll" means that the VB6 did not find the DLL in any of it s search paths. It should be placed in either System32 or the application directory. Since you already tried System32 I would try moving it into whereever the application resides. If you are running it from the IDE then it has to be were the EXE or the DLL that has the DECLARE statement resides.
For Win32 DLL you need documentation on what functions are available and then use DECLARE to make them visible. As part of the declare statement you tell it what DLL to use. This MSDN article explains what going on.
DLL written in .NET have to be exposed through COM in order for them be visible to VB6.
If you have the source good you can look through the various project and find out where the declares are for rscomclNoMsg. Hopefully that will tell you where to place the dll.
More details will help refine my answer.