If you have an jailbroken iPhone or jailbroken iPod Touch running iOS 4 or iOS 4.0.1 you will not be able to install apps using .ipa files. AppSync is a Cydia app which patches the Mobile Installation file on iOS allowing users to install .ipa files manually without having to use iTunes.
However, the older version of AppSync will not work on iOS 4 and you will have to install AppSync 4.0 on your iPhone or iPod Touch in order to directly install .ipa files on the device. You will need to add a new repository to Cydia in order for this to work, learn how to add new repository to Cydia.
To install AppSync 4.0 on your jailbroken iPhone or jailbroken iPod Touch, start Cydia and click on the “Manage” tab on the bottom. Now click the “Edit” button and then click on the “Add” button. This should bring up a screen where you can add a new repository to Cydia.
Enter http://cydia.hackulo.us in the textbox provided and click on the “Add Source” button. Now go back to Cydia and click the “Search” button at the bottom and search for “AppSync”, from the available options select “AppSync for OS 4.0” and Install it.
That’s it you will now be able to install .IPA files to your iPhone or iPod Touch without having to use iTunes, please remember that you will need to jailbreak iPhone before you can install AppSync.