I apologize I have no method to do it in the old version of Logic but on either platform there are easy, free, and good quality ways of doing this.
In Mac use iTunes. Under the File menu choose "Add to Library" and choose your wav file. Then select the file in iTunes and go to the Advanced menu and select "Convert Selection to mp3"
For Pc you might need to do some downloading. Try this:
http://www.audiograbber.com-us.net/
There are pretty easy instruction on how to use the product and also improve the codecs it uses.