Thank you very much for your answer.
I followed the instructions on the ArcGIS Help Site but I am not able to append multiple (origin and destination) locations with the "Add Locations" tool.
The workflow by creating new toolbar ("Network Analyst") and then creating "OD Cost Matrix" I can easily reproduce ..... load my origin and destination class into the OD Cost Matrix classes by right clicking into the Content Table onto "Origins" and "Destinations".
But I have to create a model in the model builder, so I need to do this with tools from the toolbox.
So I used at first the tool "Make OD Cost Layer Matrix". Then I have to use the tool "Add Locations".So at this point appears the problem that I cant load several classes (the origins and destinations classes).
In the end you have to use the "Solve" Tool to finish the process.
What am I doing wrong?
Sorry if I am too stupid but I find that this is really user unfriendly...