Were you EVER able to save in MP3 format? That is, is this a NEW problem, or have you NEVER been able to save in MP3 format? If the later, have you downloaded the WAV-to-MP3 converter? Usually the first time you try to save in MP3 format, you are prompted to download the free converter, which then installs itself, and you never have to install it again.
You can manually download the installer from here:
http://www.nch.com.au/components/mp3el.exe
Just download it to your desktop, and double-click on it. It will find your WavePad folder and install the needed components.
Jerry