Filemaker plugins are compiled libraries of functions that augment filemaker's native compliment.
I believe plugins are to filemaker, what your previous experience with DLLs are to those programs.
However, DLLs are not a valid format for "loading" into filemaker in any case.
What are you trying to do? We can probably recommend a plugin or even a native way of doing it without messing around with DLL files.
or write a custom plugin for FileMaker to talk to the DLL.
I have many sets of RFID equipment (encoders and readers) that are used for Staff Management and Access Control. These equipments were programmed by commands of the .DLL file in my old system using PowerBuilder. Now we are replacing the old PowerBuilder system with Filemaker Pro 14 and encountering the problem of integrating existing hardware with Filemaker. The .DLL file ties closely with the hardware and maybe too much effort to recreate a plugin for old equipment.