Installing AppSync 4.2 is simply the same with install AppSync 4.1 on iOS 4.1 which you can follow in the following below:
1. Launch "Cydia",
2. Tap on "Manage"
3. And then tap on "Source":
4. Hit "Edit" to "Add" the url source. Then drop this url http://cydia.hackulo.us, and then "Add Source"
Cydia will do the automated steps in order to automatically update the source.
5. When it's finished. You can search for AppSync for OS 4.2., then install the app. Now you have the ability to install .ipa files manually on your iOS 4.2.1 iPhone!
Congratulation and see you again!