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An user defined VDF is not found, in case it was referenced an error message is shown:
'Could not load DLL...'

Please check the following properties:
1) The DLL's architecture has to match to the used software, i.e. a 64 bit Visum version only works with a 64 bit dll and won't recognize a 32 bit dll.
2) The dll (for Visum 18) imperatively needs to be located under %APPDATA%\PTV Vision\PTV Visum 18\UserVDF-DLLs\.
3) The file name has to start with 'VISUMVDF...'.
4) The installed VCredist.exe needs to match the Visual Studio version, which was used to build the DLL.
VCRedist is installed together with the 'Visual C++ Redistributable für Visual Studio' package, available on the Microsoft website
- Visum 15/16 uses the VCRedist version of Visual Studio 2013.
- Visum 17/18 uses the VCRedist version of Visual Studio 2017.
5) Please use the solution in the folder c:\Program Files\PTV Vision\PTV Visum 18\Data\UserDefVDF\
Do not create any new project, use the example CPP file. Adapt the formulas needed in this file.