If you are using a Mac - try this; it worked for me:
Rather than clicking on "File -> Save", or "File -> Save As" just click on the "Save File" icon in the task bar. When I did this WavPad automatically downloaded and installed the missing m3pel codec - and then saved the file as an MP3. This is assuming you started with an MP3, of course.
(For some reason they didn't include the codecs for the Mac OS version in the download so the drop-down menu doesn't have the option of "saving as" MP3 - for example).
