You should talk to the people who created it - they should provide technical support and will know more about their product than we will. If they don't, then find another supplier and demand your money back!
As Dave says, if the DLL file is not a .NET assembly but Native code then you cannot add it as a reference, you instead have to use P/Invoke:
Platform Invoke Tutorial (C#)[
^].
That's not necessarily simple: you need to be very careful to manage datatypes in particular to make sure that what you pass to the API functions are what they expect - so read both the documentation of the functions and the tutorial material very carefully.
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