Hello and welcome to the forum.
After you have Registered the hardware the software should be listed in your USer Area / My Products section. You may see "AmpliTube Custom Shop" listed too but do not use that Serial Number, as that one unlocks only the free gear.
if not there, have a look in the User Area / Promotions section.
Otherwise, please contact
Sales/Account Support, via here. You will be able to search the FAQs first, but if they do not resolve your issue, there is a link at the top to a Support form. Sales Support can look at your Account and see what's going on. Response time normally is within 2 working days (US (East Coast) Business hours, generally), but in the current circumstances please allow some more time. Administrators and Moderators here cannot access your Account.