Your library is tied to your free YouCanMakeThis.com user account. To have the files from your purchase automatically added to your library, make sure you are signed in to your account prior to checking out. Then, after your purchase your files will automatically be added to the My Library section of your account, which you can access while logged in.
If you forgot to sign in to your account before checking out and would like your purchase added to your library, use the support request form on the Contact Us page. Provide the date of your purchase, the Order Number, and the email address you use to sign in to your account.