Some apps are made to be only available for the newest 2.2 firmware. But don’t worry, you can get it to work in 2.1 firmware using the method I’m going to tell you here.
NOTE: PLEASE FOLLOW THESE STEPS CAREFULLY OR YOU WON’T BE ABLE TO REBOOT YOUR IPHONE!
1. Download the FWChanger from Cydia. Use the search menu then write “FW”.
2. Then, open the app.
3. In Firmware section, write “2.2” and in the Build section, write “5G77”.
4. Then, click “Set Custom Firmware”.
5. If everything is ok, you will be able to install 2.2 apps to your 2.1 firmware.