Most likely, you simply haven't activated support for the MP3 format in the program yet. Therefore, import/decoding and export/encoding operations in this format are not possible yet. You can activate MP3 support by obtaining the "More tracks & formats" feature package in the "More features" category of the built-in store system. If you're using the "Plus" or "Premium" edition, you will be able to spend one of your feature credits on this. If you're using the completely free version or one of the smaller content edition of Music Maker, you'd be able to buy this add-on feature at the price posted in the store.