I'm having problems with Installous on my iPod Touch! PLEASE HELP!!! :(?

I have an iPod Touch 2nd Generation. Today, I jailbroke it using GreenPoison. Everything worked fine, but then I got Cydia. I was told that I was supposed to download installous from Cydia. That's what I did. I was told that once your done with that, when you open Installous there should be a search bar where you can search for any applications. Instead, I get this message, "Nothing here? Use Safari to download/install .ipa files" I have no idea what .ipa files are and I have no idea where to get them from! PLEASE HELP!! :( I'm dying here I've been trying to do this for hours!! :(