I had the same problem with a different DLL and the problem was due to the way Zemax installs with duplicate folders.  I've folders C:\ProgramData\Zemax\DLL, C:\Program Files\Zemax OpticStudio\DLL and C:\Users\Andrew\Documents\Zemax\DLL.  The correct folder turned out to be the C:|Users one. To check that you are looking at the correct folder just delete one of the DLL files.  If the DLL does not disappear from the drop down list you are looking at the wrong folder.

If anyone from Zemax reads this please can you stop this idiotic duplication!

Andrew

Hello Andrew and Chiara,

I would recommend looking in the 'Folders' tab of the 'Project Preferences' to see the correct place to store files.

As for the the different places to store files, after version 17 came out, all the DLLs and other files were moved to users\documents\Zemax from the program files location. I imagine the newer version were downloaded over the old version and that is why it looks like there are duplicates of all the files. This knowledge base article discusses the file location in the {DATA} folder: https://my.zemax.com/en-US/Knowledge-Base/kb-article/?ka=KA-01532#addendum. 

-Kaleb

Dear Andrew and Kaleb,

Thank you for your answers, I figured out which was the correct folder and I managed to load the dll.

Best,

Chiara