I can not find previous purchases

If you are trying to see your purchase history then you should be able to do that on your computer's iTunes via the Store > View My Account (Store > View My Apple ID on iTunes 11) drop-down menu.

If you are wanting to re-download past purchases then not all content types can be re-downloaded in all countries (e.g. music, films, tv shows can't be re-downloaded in all, and audiobooks and ringtones can't be re-downloaded anywhere), but what you can re-download whould show in the Purchased tab in the iTunes and App Store apps, and via the Purchased link under Quick Links on the right-hand side of the iTunes store home page on your computer's iTunes : re-downloading.