Okay, I'll give you a step by step on how to use this thing. BTW, it would be easier if we could do this on msn chat (or aim). Let me know if you have one of those.
Anyway, here's how to basically use cdex.
If you're getting an error message everytime you start it up, try putting a CD in your CD rom drive
Start up cdex (I just realized how old my version is!)
To get your mp3s at the 128kbps, click on options, then go to settings
Under the encoder tab, change min bitrate to 128kbps (unless it's already selected)
Click OK
Click on Convert, and select WAV ->MPEG
Click on the button marked ....
Choose the directory your .wav file is stored in
Click OK
You should now see your wav file in the file list
Highlight the file, and click "open"
The file should start converting.
Let me know if this works for you.