While I never used a Labview DLL in a VB6 project, I know that a LV DLL, unlike a VB6 DLL, doesn't have a COM interface and therefore must not be registered.
What you need to do is rather like calling a VC++ DLL, using the Declare statement with the Lib specifier.
Paolo
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LV 7.1, 2011, 2017, 2019, 2021