Hi Tina,
Once you have purchased AutoCollage from the online store, the registration key will be placed in your digital locker. If you purchased from the Windows Marketplace in the US, then there is a link to open it at the top of the page. If you purchased from the Microsoft Store in the EU then you should click on the "Account" link in the top left.
In either case it will ask you to login and will then display any products you have purchased along with the registration key.
This key can then be entered into AutoCollage by clicking on the "Options" button, selecting the "Registration" tab and then clicking on the "Register" button. This will open a new window where you can enter the registration key.
Hope that helps,
Thanks for purchasing AutoCollage!
Chris.