Launch iTunes. Click the "Edit" menu, then click "Preferences." Click the "General" tab.
Click the "Import Settings" button. Select "MP3 Encoder" from the "Import Using" menu. Click the drop-down menu next to "Setting" and choose "Good Quality," "High Quality" or "Highest Quality." The higher the quality setting, the more space the converted MP3s take up on the hard drive. Click "OK," then "OK" again.
Click the "File" menu, then "Add File to Library." Navigate to and double-click the first ALAC file you want to convert. Repeat the process for all the ALACs you want to convert to MP3s.
Press and hold the "Ctrl" key. Click each one of the ALACs in the iTunes Library. Release the "Ctrl" key.
Right-click one of the highlighted ALACs. Click "Create MP3 Version" in the pop-up menu that appears. When iTunes finishes converting the ALACs to MP3s, press the "Delete" key to remove the ALACs from the Library, if you wish.