Not sure how to answer the first part of your question.
As for adding an mp3 or any audio file, option-click to download
this short tutorial movie. It shows how to add audio straight from your iTunes library. You can also simply drag audio clips into your project (where the video clips normally live) and then drag them down into the movie timeline. In other words, you can treat them just like video clips. Just remember that there are two 'views' of your iMovie project 'Clip View' and 'Timeline View' which are toggle by the two buttons to the bottom left of the movie window. If you put your project in 'Timeline View' you will see that you have 3 tracks (1 video, 2 audio) that you can simply drag-and-drop what you want, where you want. If you are looking at your project in 'Clip View', you won't see that you have these 3 tracks available.