Why can't I install my Installous downloads on my iPhone?

I have an iPhone 3Gs that is jailbroken and unlocked. It is running on iOS 4.0.1. I installed the Appsync program and have Installous 4. I download my files fine, but when I go to install them an error box pops up and says "Invalid IPA". I have searched the internet for ways to resolve the issue and haven't been able to find anything helpful. I've reinstalled Appsync. I have reinstalled Installous. Can anyone help me fix the problem?