Instagram problems with ipod?

[Amber]

Okie. So today I downloaded instagram to my ipod 4g. Once it started downloading, it stopped halfway through installation. Since I waited like 30 minutes, and wouldn't continue installing, I deleted the app. So when I went to redownload it, the place where you choose to download the app says "open". and usually when it says that it means you already have the app on your ipod, right? but, since I deleted it, I have it no more. So when I press open nothing happens. It should say download! I turned my ipod off and on like 5 times, but it still syas open instead of download. Please help! (sorry for long and complicated question.)