Go to Cydia, add the source cydia.hackulo.us, accept the source when it asks you if you want to add it (the warning about it distributing copyrighted apps) go to search and look for AppSync and install it. You might need to reboot, I can't remember if it ask you to then do so.
You can now sync cracked apps, search them at
appulo.us for the app you want. If you want to add them straight from the site to your iPhone then also search for and add Installous, if you add apps through Installous they won't show up in iTunes however so I just download them and add them through iTunes. Once you download the .ipa simply double click it and it will be added to iTunes, then sync.