I know I'm answering my own post but...
I just found the templates in the download centre under extras, serves me right for not looking properly!
Although any help on the subject would still be very much appreciated.
It may be that DEP is your problem on Windows 7. Check out this link for how to turn DEP off for ProcBook.exe: social.msdn.microsoft.com/.../ece9ca80-6c83-4ce4-a7a2-7122acf68d47
The down-side is that you are disabling and operating system security feature, but it can often be a quick fix for VB6 code.
If you have the source code for the add-in, recompiling should be as simple as openning the project, deleting the references to the PB libraries (PBObjLib, PBSimLib, etc), and then adding the PB 3.3 versions from %PIHOME%\ProcBook\Public Assemblies.
As you posted, the add-in templates can get you started in one of several languages. It's going to be easier to port VB6 code to VB.Net, so I'd start looking there. But if you really want to take your add-in into the 21st century, then C# would be the way to go, and there's an example for that too.
Also, if you provide more information about what is going wrong with the VB6 add-in, we may be able to figure out how to get it running. We try as hard as is feasible to keep PB backward compatible, so we'd be interested in anything that works in PB 3.1/3.2 but not in PB 3.3.
It would appear your first suggestion did the trick, the add-in works correctly with the new references. Thanks for your assistance.
I may still try to either port the code to VB.Net or re-create it in C#, just to make the add-in a bit more future proof (hopefully).