Please follow below steps to add songs to Couch to 5k or 5k to 10k app music function:

IOS Devices:
  1. Open app
  2. Go to training part
  3. Select one workout
  4. Click Go To Workout
  5. Click music signal at right bottom of screen
  6. Click Playlists 
  7. Select desired playlist
Note: Please make sure you have already created a playlist in iTunes                                                                                                           
Android Devices:
  1. Create new playlist​​
    1. Go to app Settings > Workout Options > Playlist > Add
    2. Create desired playlist
  2. Add songs to existed playlist
    1. Go to app Settings > Workout Options > Playlist 
    2. Click triangle icon on right side of desired playlist
    3. Click Edit Playlist to select local songs to add to playlist