If you are uploading ENHANCED audio podcasts to PodcastMachine - (that's podcasts created with Garageband and with the filetype .m4a) - the you should submit the AAC feed to iTunes since this is the only feed that will let you download the enhanced podcasts. The 'ipod' feed normally used contains Mp3's instead of the enhanced version. 

To submit the feed in iTunes, click on this link https://phobos.apple.com/WebObjects/MZFinance.woa/wa/publishPodcast and follow the instructions.