When did you jailbreak? Because version 1.13, out since around late November IIRC, handles fw5.6.x just fine.
If earlier, then all is not lost, you can still use hacks but it will take a little more work. Reread the OP, specifically the red "Even more important notice regarding fw>=5.6.0".
Also
make sure KUAL is at version 2.5 (else the updated jailbreak with gandalf and all doesn't get used...), and check out the MobileRead Package Installer.