At the first start the plugin checks whether you had installed MP3 support. It tries to play the file empty.mp3 and if it fails it automatically install the required packages. I've copy this code from the Rhythmbox Ubuntu One Music Store plugin.
In my opinion it would be better if you select the panel entry MusicStore the panel is resized to the whole window size. If you select another entry the panel should resized back.
But I don't know how to manage it.