I get this Invalid IPA error everytime I try to install something in installous 4?

I have the latest ipod touch with the camera I think it's like 4.2.1 and whenever I try to download something, it will download but when it's installing, it gives me the Invalid IPA thing. I already tried uninstalling Installous, I have AppSync 4.0+, and then I tried that sinfulrepo one too.. I'm kinda upset that I just wasted over an hour downloading this 500mb game and now it won't install. Please help!!