You can find them on your hard drive in explorer, right click on them and (if you have "enable shell integration" selected in options) click "Media Center" then "Import". You should then be able to see them in MC.
Or click on "Library" then "Import" and navigate to the folder.
I'm assuming you hadn't imported them. If you were trying to import them but they weren't showing, maybe make sure that that file type is being looked for (in the import window, on the file types list).
Alternatively do everything in MC from the start. Click on audible in the Services and Plugins section and login and redownload the files. MC automatically puts them into its libaray that way.
Once they are in your library, it's simple to drag and drop them onto your ipod. Or you can put them in a playlist and synch that playlist to the ipod.