Just follow these steps
1. Open cydia
2. Goto Manage tab below
3. Tap on Sources
4. Tap on Edit
5. Then tap on Add
5. Then go to Sections tab6. Open All categories
7. Scroll down and find the following package AppSync for OS 3.0 and install that package.
8. Restart your iphone.
9. Install any one free app from appstore
10. Now you are ready to install any cracked app