Successful opening in Creo 8.0 is actually good news: you have a working environment.
If we stick to the meaning of the error message, the application RealDWG needs to be installed. But can confirm that Creo 8, Creo 9 and RealDWG are installed on the same workstation?
@TomU is right.
The article and a deeper search over the internet confirm that this is a third party that Creo OEMs.
Let me roll back a little:
If yes, then either :
All in all, in order to test bullet 2, can you confirm the presence of that executable in both installations?
I just checked my server and pro_dwgconv.exe still exists in Creo Parametric 9.0 but the location has changed.