In the Window's Explorer, select the mp3 files you want to convert.
You can select the mp3 files by left clicking and then dragging over the file icons (When you are dragging it shows a big square.). Or you can select the mp3s by holding down the Ctrl key and left clicking on the individual songs you want.
Right click on one of the selected mp3 files and a menu will appear. Click "Convert mp3(s) to wave(s)…" and the program will start! All the files selected will be queued up.
If you are still not quite sure how to start it, here is a highly compressed realvideo (142KB) which shows how to right click mp3s to start the conversion process.