create audio loop?

There are different ways to do so, but I don't know if they are the most efficient in your case.
I still believe GarageBand for Audio Loops is the best solution and it's easy to do.
In any case, if you want to do it in within iMovie, know that you cannot copy/paste audio tracks.
So, you will have to repeat these steps until you achieve the total length of the loop you are looking for
1. In View Menu enable Playhead Info
2. Import your mp3 file in iTunes (this way it will be seen by iMovie Music and Effects Browser)
3. Create a Project of the same length of your mp3 file by adding non-audio clips, i.e. backgrounds, maps, whatever...
4. From the iMovie Music and Sound Effects Library drag and drop your mp3 as a voiceover file (it will show underneath your clips as a green track, like this) and
5. click-drag the end of the green track toward the left in order to trim it to 10 seconds



Don't forget to adjust your audio fades/levels in Inspector.

If your loop is in your iTunes Library, you can simply drag and drop your audio track as a Voiceover track to your iMovie project directly from the Music and Sound Effects Browser to the right of the UI. (Click the Note Icon)
You can drag and drop your audio track also as a Background Audio.
If you need 30 seconds of it, you keep adding that track at the end of the previous one. If you want to prevent the audio fade-in/out (which is automatically set to 15 frames)
Go to Inspector, click Audio tab and zero the Fade-in/out value.
Or you can create your loop into Garageband and then save it enabling optimization for iLife '09 so that your song will be seen and understood by the iMovie Music and Sound Effects library. In any case, you can find out more about how to add audio to your project by visiting the tutorial videos web pages here:
http://www.apple.com/ilife/tutorials/#imovie-music
http://www.apple.com/ilife/tutorials/#imovie-voiceover
http://www.apple.com/ilife/tutorials/#imovie-sound