Maithili,
You will need to get the class pointer from the DLL first as a value, then pass it back in. You cannot just generate a pointer value in the LabVIEW execution system and expect it to work in the DLL execution system. Remember, the pointer is just an I32 value.
Another thing you could try is to have a wrapper DLL that can store that information and pass it to your original DLL.
Randy Hoskin
Applications Engineer
National Instruments
http://www.ni.com/ask