Hi Nenad,
[quote user="Nenad Poljcic"]How do I activate existing DgnModel using NET API c# ?[/quote]
There is an open Enhancement Request # 432451 for a .NET method to activate a loaded model.
Known workarounds are to import the mdl API function mdlModelRef_activateAndDisplay or the COM API method Activedesignfile.Models(modelName).Activate
Best regards,
Artur