When I try to reference wwctl64i.dll, I get "Invalid extender" ....
You can only us a 64-bit extender with 64-bit WinBatch. The bitness of the extender must match the bitness of the executable. That is why the 64-bit versions of WinBatch extenders were created in the first place.
Ok I get it. I am used to just double click my WBT files.
Change the script file name's extension to '.wbt64'.