Install Cracked Apps on iOS 4.2.1 with AppSync 4.2

The most popular tweak in Cydia, AppSync is now available for iOS 4.2.1. It lets you install cracked apps on iOS 4.2.1 firmware by patching the MobileInstallation file on iOS 4.2.1. Once you have installed AppSync iOS 4.2 on your iDevice running jailbroken iOS 4.2.1, you’ll be able to install cracked apps (.IPA) on your iDevice without using iTunes.

Your device must be jailbroken on iOS 4.2.1, use our guide posted here to jailbreak iOS 4.2.1 devices with the latest version of Redsn0w then tap to Cydia and add as a source then search for AppSynce for iOS 4.2 and install it.