First, confirm that the content is available for re-downloading in your country:
http://support.apple.com/kb/HT5085
Then make sure you're looking in the right place:
http://support.apple.com/kb/ht2519
If you confirm that it should be available and that you're in the right place, post back and we'll go from there. We'll need to know your country, your version of Mac OS X, and your version of iTunes, for starters.
Regards.