AFAIK, you need iPhoto on a mac. You'll probably have to sideload the photos onto your iOS device and let them sync that way. Remember though, photostream is only a temporary place holder for your pics (and now videos with the new iPhoto update). But they will get deleted after 30 days.
Its main function was designed to let you sync photos back to your other devices once you take them on your phone or ipad. Then you move them into iPhoto or a folder for long term storage. On your device, you must move them from your photostream folder to the Camera Roll to keep them forever (iCloud will back them up forever if you have the space, but they will disappear after 30 days in Photostream). Now that Apple TV has Photostream for viewing and/or the screensaver, 30 day limit is pretty lame.
Very stupid way of doing it if you ask me. Apple should just let you "photostream" all your photos indefinitely... it doesnt take up any more space on iCloud!