You can download and install any release of MATLAB from mathworks.com. If your MathWorks account is linked to your licensed information, click on "My Account" on the top-right of the MathWorks home page and then on "Get Licensed Products and Updates" on the left panel. Otherwise, you can contact your MathWorks account manager to obtain access to a licensed account. You may also contact the MathWorks Installation and Licensing Support to see if there are alternate options available.
Regarding the DLL-file - is this a MEX-function to run in MATLAB? You will need to contact the author for that. You could potentially find contact information on the page you downloaded it from. Note that although MEX-functions with the DLL extension are not supported on newer releases of MATLAB, you might get it to work on a 32-bit PC version of MATLAB. See Running MEX-Files with .DLL File Extensions on Windows 32-bit Platforms.
