There are several reasons why you receive the 80040154 Class not registered error. The include not installing the SDK files and using the wrong QWC file.
Make sure you have installed the SDK:
For the POS SDK, the user name on the web connector is test and the password is password. If you are able to see an app in the web connector, then the SDK is working.
There is a pairing file for QB Desktop and QB POS. Make sure you have downloaded the right one: