I hope you have access to the White Papers. I just checked the OPC Foundation site and they have stopped sharing them. I would give you what I have except all I can find is the hardcopies right now. Anyway, you should have the OPC White Paper called "Data Access Automation Interface Standard" mine is version 2.01.
Attached is the first and only VBA app I wrote for our OPC client. It's very simple, let me know if there is anything I can do to help.