By default, Pro Tools LE does not include .mp3 as a supported file type for importing or bouncing/exporting. This option can be obtained in two ways:
1. By purchasing the Music Production Toolkit. This software toolkit contains several plugins for Pro Tools, the bility to use 48 voices (instead of 32) in a session, and the ability to import and export .mp3 files. You can purchase the MPToolkit here: https://www.sweetwater.com/store/detail/MPToolkitLE Note that the MP Toolkit requires the use of an iLok for authorization.
2. By purchasing the MP3 Option from the DigiStore. This is a less expensive alternative to the Toolkit, and only gives you the ability to import/export .mp3 files, without the extras. This is an option that must be purchased directly through Digidesign. You will get instructions via email on how to download, install, and authorize the MP3 Option. Note that you can use either a challenge/response system or an iLok for authorization. You can purchase this option at the DigiStore: http://store.digidesign.com