It looks like you are using a MEX-function which used to have a .dll extension in very old releases of MATLAB. They now have extensions like mexw32, mexw64, etc. You can call these files directly like any other MATLAB function - there is no need to use LOADLIBRARY.
In any case, the issue is that MEX-files are platform-specific - the one you have is compiled for 32-bit Windows, so it won't be recognized on 64-bit MATLAB. If you have the source code corresponding to the file, you can use the MEX command to re-compile it for 64-bit Windows. Alternately, you can install 32-bit MATLAB on your 64-bit machines to run the 32-bit MEX-file. See How can I install 32-bit MATLAB on my 64-bit Windows machine? in matlab