Vista - Error when loading .DLL as a reference

It seems that I'll need to call S-PLUS functions from my C#.NET code.
I've learned that it's only possible to call S-PLUS from C++. However,
someone told me that he was able to call S-PLUS functions from his
managed C++ code. Therefore, he concluded that I should be able to
call S-PLUS from C#.NET.

I've finally found "sconnect.dll" and decided to add it as a
reference. However, I got an error when adding the reference:

A reference to 'C:\Program Files\Insightful\splus62netclient\sconnect
\release\sconnect.dll' could not be added. Please make sure that the
file is accessible, and that it is a valid assembly or COM component.

I took a look at its property, the file was created in December,
2003.

Any advice on why it doesn't allow me to get it added as a reference?