You can find a description with pictures in the User Manual of SongBook+ in the chapter "Add PDF and MP3 files to SongBook+ using iTunes".
Here is a short summary:
- Connect the iPad to your computer.
- Start iTunes.
- Select the small iPad icon in iTunes.
- Select "File sharing" on the left side in iTunes.
- Select the icon of SongBook.
- Use the button "Add...", to add your documents.