There is no correct way to use Modelica.Fluid in OpenModelica as it doesn't yet support the whole of Modelica.Media.
As for SimForge, I have no clue

Thanks for your reply.
But even if I would try to simulate an example from another library, eg thermal or electrical. 

Would the simple model as proposed in my previous post work?

and if OpenModelica does not support the Media, does it mean I cannot use the standard library components that make use of a medium? All components that are interesting to me (HVAC systems) use a 'medium'...

thanks,
roel

and another question: where can I find CLEARLY what OpenModelica supports, and what is NOT supported?  (In terms of Modelica language, standard library versions, ...)

thanks,
Roel

Defining what OpenModelica supports or not is complicated, because everything is divided into frontend and backend. We can flatten most code since the people funding the project think that's most important (but this means we can't simulate all code we support).
You could try looking in: http://openmodelica.org/svn/OpenModelic … istration/ (login using the subversion password http://www.openmodelica.org/index.php/d … rce-code).
The simple answer is: Modelica.Media, Multibody 3.1, Multibody 2.2, enumerations have the poorest support.

As for SimForge, I think you're on the right track but since I find command-line OpenModelica works better, I'm not really qualified to answer questions regarding graphical model editors.