I don't know if this helps, but I've had success using my own music by creating playlists.
- Synchronise your local music with the desktop application. If your music isn't in one of the usual places (iTunes, "My Music" folder, etc), you'll need to "Add source..." via your Spotify preferences (CTRL+P).
- Create a playlist on your PC, the playlist can include local music and/or Spotify tracks.
- Link your Spotify Mobile app to Spotify Desktop
- On iOS, you can do this by connecting your iPhone to the network your PC is on (i.e. over WiFi), or by connecting your iPhone to your PC via USB
- On Android, you'll need to use WiFi.
- Open the Spotify app on your mobile device. Once you do this, wait a few seconds and your phone should appear under "Devices" in the Spotify desktop application (left side, near the top).
- Once your device appears, you will be able to see a list of all your playlists.
- Find the playlist that contains your local music, and make it available offline. Spotify will then sync your local tracks onto your mobile device.
As far as I'm aware, you can't directly sync your local MP3s onto your Spotify mobile app, you have to do it through the desktop app. I could be wrong though.
Hope this helps