It sounds like your LabVIEW code is successfully calling your .dll It is then the .dll's responsbility to find the files it needs, and ideally the documentation with the .dll or with Matlab should answer this question. After LabVIEW calls your .dll, the code in the .dll is the code in charge of what happens. There is a good chance that the .dll is only programmed to find the files it depends on at a given relative or absolute path which is no longer correct wherever you have copied it to.. if you control the code for the .dll, particularly the error message, this is a good reason to have the file not found error message explain where it was trying to look (i.e. be specific)
If this program is something you just need to run and not distribute, maybe you should just
browse for the .dll from the Call Library node in the location where Matlab creates it... if it doesn't even work there, you will definitely need to find out how the Matlab .dll is supposed to work.
If you can call a .dll from Matlab, try moving it and then calling it from Matlab again, and check whether any Matlab environment path settings are being used or assumed in the .dll code.
Good luck
