Downloading happens on the web app at app.digitaldjpool.com, not on the mobile app.
The mobile app is a companion — it's built for browsing the catalog, previewing tracks, and bookmarking anything you want to grab later. Think of it as crate-digging on the go.
When you're back at your computer, open app.digitaldjpool.com, head to your Library → Bookmarked tab, and download the tracks you saved. Everything you bookmarked on your phone is waiting there.