Depends. If the DLL is a plug-in, then you don't need to do anything except place it within your compiler\plugin-users directory in your DBPro installation directory.
If it's not a plug-in, then you need to use LOAD DLL/CALL DLL to access what it provides.
As for knowing what/which commands are available and how to access them, then you should also have help files provided which show you that.
Unfortunately, it appears that BBB.dll doesn't have help files, but it does have a full set of example programs that you can look at and play around with - they look fairly self-explanatory.