Hmm, I don't really know .NET, but have you referenced zlib properly?  In VB6, you use Declare Function ____ Lib "zlib.dll" to declare an API function.
Since I'm guessing that the compiler isn't complaining, I'm presuming you've referenced zlib correctly.

I'm guessing the problem is that things aren't being passed by reference/value properly.  Does .NET have the VarPtr() function?
You may need to double-check your API declaration to make sure that everything is passed by value or reference where appropriate.

If you're really stuck, unfortunately, I can't help much (since I don't really know .NET).  I'd recommend looking for some zlib handling for .NET examples - they should help you figure out what's needed.

Anyways, best of luck on your project, pka4916!  I'm glad you've decided to use my code :)