You can convert the audio to the correct format for FCE by using Quicktime 7.
Open the MP3 file in Quicktime 7 and then go to the menu File > Export.
From the drop down menu on the Export pane, choose Sound to AIFF.
Click the Options Button and once the options pane opens choose:
Click OK, then save the exported file to a location of your choosing.
Import the file you just converted into FCE and you should be good to go.