On left fourth on list says import audio or music. Click that then find your song in the search field. No?
May help I hate Itunes. But anyways, I believe Movie Maker doesn't recognize the file type. It accepts WMA or MP3 and maybe some others. You can try that link or download some free software to convert the song into another file where you can then import it into Movie Maker.
You can add content to your Windows Movie Maker project either by capturing content directly in Windows Movie Maker or by importing existing digital media files. The content you capture in Windows Movie Maker is saved in Windows Media Format. However, you can also import existing Windows Media-based content as well as other common file formats.
You can import files with the following file name extensions into Windows Movie Maker to use in your project:
Audio files: .aif, .aifc, .aiff .asf, .au, .mp2, .mp3, .mpa, .snd, .wav, and .wma Picture files: .bmp, .dib, .emf, .gif, .jfif, .jpe, .jpeg, .jpg, .png, .tif, .tiff, and .wmf
Video files: .asf, .avi, .m1v, .mp2, .mp2v, .mpe, .mpeg, .mpg, .mpv2, .wm, and .wmv