Yes you read that right ! AppSync for iOS 4.0 (formerly iPhone OS 4) is now available for all those of you who have managed to jailbreak your iPhones and iPod touches. For those of you who don’t know, AppSync patches MobileInstallation file on iOS which is needed in order to install .ipa files manually without using iTunes.
Make sure you have jailbroken your iPhone

or iPod touch on firmware 4.0 before following the steps below.
Step 1: Start “Cydia” on your iPhone.
Step 2: Touch on “Manage” tab on the bottom.
Step 3: Now touch on “Sources” as seen in the screenshot below:


Step 4: Touch on “Edit”, and then on “Add”. You should now be prompted to enter a url source as seen in the screenshot below. Type “http://cydia.hackulo.us” and click on“Add Source”.

Cydia will now automatically update your sources by following a series of automated steps as shown in the screenshots below:
Step 5: After installation is completed. Search for “AppSync for OS 4.0” in Cydia and install this application. This app will automatically patch mobileinstallation file on your iPhone or iPod touch running firmware 4.0.

You should now be able to install .ipa files manually on your iPhone running firmware 4.0.