Click on the insert tab, then on the far right, click on audio. Then click on audio from file on the drop down. That will bring up a window that lets you browse to the folder containing your mp3(the window is embedded in image below). In the bottom right you can even change it from audio files to just show mp3 audio files. Click on the drop down arrow beside insert and make sure you click insert and not link. If you do all that and insert the audio does it give you an error on playback?