AppCake is useful for installing Apps on your jailbroken iPhone if you run into trouble with the Appstore. Of course you should not pirate apps with it, but for me it was a useful legal savior here and there for some problems.
Unfortunately it shows Error Codes, which are not understandable for the normal human.
If you encounter a problem while installing an app in Appcake with ErrorNo. -999, just search Appsync Unified in Cydia and install it.
Now the problem with Errorcode -999 should be fixed and you should be able to install the app.
I also found a little video on Youtube where some one is visualizing this process via Apple iPad simulator.