I believe that .dll (Bigdll.dll) was created by the 3DGS forum user TripleX (check name).
There should be a thread for Bigdll, in the 'Contributions' section of this forum.
You should be able to find the latest version there.
*I would not attempt to use Bigdll with A7 and / or Lite-C without more information.
I think 3DGS versions 6.31.4, 6.40.5, 6.5 (buggy) and 6.60 are OK for Bigdll, but check that or experiment.
If you are using C-Script .wdl, you should have what I refer to as a 'main' file, a .wdl which contains function main.
Try to 'include' your scripts in that file before functions in that file.
Try to add this line near the top of the 'main' .wdl and place Bigdll.dll in your project directory (folder) not the work directory:
plugindir = "."; // look for and automatically open plugins in current project directory
With that, you shouldn't need to explicitly open the dll with the dll_open function.
So you would probably nix the starter terminate_file and the dll_handle = dll_open(...) line.
The Bigdll plugin distribution should contain a .wdl 'interface' file (FreeDll.wdl?) with dllfunction C-Script declarations.
Place that .wdl in your project directory (the directory containing the 'main' wdl) and 'include' it in the main .wdl file near the top, before functions, maybe right after the plugindir = line:
include <FreeDll.wdl>;
Then you should be able to use the functions declared in FreeDll.wdl (check name in newer versions). There should be brief instructions in that file also.
If the .dll is opened or accessed 'automatically' and 'correctly' there should probably be a brief message as an indication in the white startup window when you start the engine.
Normally, you do not have to 'register' 3DGS .dlls, I think.
I recommend that you do not use the work directory. Use a separate project directory and sub directories in that directory to organize assets.
If you are using an older version of 3DGS, some plugin .dlls, like Bigdll.dll may not work.
I use 3DGS version 6.60 and work with .wdl on occasion (no A7 or Lite C yet). In the past, I have used the 3DGS C / C++ SDK to write a few plugin .dlls. There are some questions, I can not answer, or will not answer. There is some help, I can not offer, or will not offer. However, if you need a bit of free, minor assistance,...