You should have got DVD Architect Studio 5 with your purchase of Sony Movie Studio 12 Platinum.

Check your (download) order confirmation email/ (physical copy) package  for the serial number.

This link should have been in your email

https://www.sonycreativesoftware.com/download/link?id=7630.1

The download link on http://download.sonymediasoftware.com/current/ still should work.

Note this post from MAGIX_Eric D from 09/06/2016

MAGIX is currently implementing a web page which will allow downloading the former SCS products, and an activation system which will add the serial to your account, and activate using a MAGIX server.  This is currently not implemented as of yet but will be up and running in the near future.  In the meantime the download links on the SCS side are redirecting to the product pages on the MAGIX side.  Thank you for your patience as work is still being completed on this. 

Meanwhile you might see if this workaround (customized for DVD A S 5) would maybe work, if you have trouble now registering/activating. Back-up your registry as advised in the link before you try.

https://www.vegascreativesoftware.info/us/forum/activation-of-new-purchase--103159/