There's a good reason we created Juno as a native app for your iPhone or Android smartphone. This means you can not access Juno from your computer (you can create your memory book there, though). What about the photos or videos you didn't take with your smartphone, but with your digital camera, and therefore have them stored on your computer? There are some simple ways to get those to your memories in Juno:
Send them to your smartphone via Airdrop (iOS), sync them to a folder on your smartphone, save them on an SD card you put into your Android smartphone or use the respective cloud service of your operating system (e.g. iCloud).