By default Apple's iPod music player streams music. This causes problems if you ever want to listen to your music when you do not have a network connection. Also Apple does not provide a way for apps like 'Slow Down Music Player' to access streaming songs or force them to down load them for you.
There are several ways to force all your songs to download to your device. See google force all my songs to download The problem with downloading all your songs is that it they can take up a lot of space if you have a large library.
An alternative is to just cause the songs you want to use in 'Slow Down Music Player' to download and then import. Tapping on 'Down loaded' will show you want songs are already available to import into 'Slow Down Music Player'. If want to import a song that has not been download, find it in your library and tap on the cloud icon