You can't that's the real crap thing about Itunes. Purchases use applces own format that doesn't work with other manufacturers.
or an upgrade? http://www.tech.co.uk/gadgets/portable-audio/digital-audio-players/news/new-creative-zen-player-designed-for-itunes?articleid=1523037078
There is a long winded way to do this..... 1. Burn your songs onto an audio CD 2. Import the CD back into iTunes 3. Then iTunes will let you convert them to MP3 Its a pain but I have had to do this myself recently.