Hi NBChaaben,
Could you elaborate on what you mean by "working directory"? This error means CVI could not load the specified assembly. It could be a corrupt dll issue.
Can you run a simple application running this .net dll? If not, then there is probably something wrong with your dll. I've included an article on using DLLs in CVI.
http://zone.ni.com/reference/en-XX/help/370051M-01/cvi/programmerref/rules_for_using_dll_files/
Hope this helps.
Josh L.
Applications Engineer
National Instruments