Under a MAC OS environment, you cannot install over current files, it generates a copy file with a number associated with it.  If you read the Readme carefully, further down the page, the vendor specifically states to copy the folders/files to your DS Library folder.  Only in Windows can you install files/folders over each other, they will either replace similarly named content or merge with the folders.  This is strictly an operating system problem and quite different between Windows and Mac.  Your only solution is to copy those content files/folders to their rightful location in your DS Library so that you can see them in your software.

Try to post a screencap of the ZIP file content and your main DS Library folder where you want the content installed, preferably the folder that DS is associated with.  At worst, you can unzip the Big Dog file into a separate folder called HiveWire Big Dog and link it to your DS software.  This is provided that you have the knowledge to link separate libraries to DS.