I use WavePad every day, but primarily with .wav files. While it would seem that your project should be very simple, I admit to having a difficult time myself working with mp3 files in WavePad.
I have done some trial and error to try helping you out, and without getting into too much detail about it all, here is what I would suggest…. (I work on a pc)
Open WavePad. Drag the original full length file into the project window. Place the cursor at the point you wish to begin your soundbite, and left click and drag to the beginning of the file. Then delete the selected part. Next do the same at the end of your soundbite, dragging and deleting everything that follows. Now – most important – save this new soundbite file with a new name, so you do not alter the original! (File menu > Save As) I would recommend saving with the attributes of the original file. You should be able to attach this file to an email.
Hope that works for you.