Firstly, that message popped up because your preorder code merely entitled you to early access and the special preorder bonus items, it is not the actual SHB expansion game code that is completely seperate and you get with your actual copy of the game. So the message to register SHB is correct.

Secondly, I don't know why you had such trouble registering SHB as for me there was a shortcut link to the specific place to register the expansion on the main account page on the Mogstation (it was a link that said literally 'Register expansion code'). Clicking it took me straight to the required place and the expansion was registered within a minute.

As for the problems with adblockers, the problem is caused by the fact websites these days run 'scripts' in the background that handle various functions, one such function on the Mogstation is security scripts to ensure your payment details are correct and not stolen. Unfortunately adblockers like Noscript cannot tell these benign scripts from malware and so throw up errors/block them from running by default. You have to manually whitelist the specific scripts to ensure the site isn't broken or else disable the adblockers completely. It's annoying, but it's a necessary evil I'm afraid.