Just wanted to comment my experience with "jailbreaking" my kindle keyboard (K3)
I ran into the issue which many users have been experiencing recently on this thread, going in a constant cycle of factory resetting, installing jailbreak, installing MKK(devCerts), installing KUAL, and receiving the "This item is not signed by a registered developer".
My kindle(k3) was running the stock firmware (3.0.2.x) when doing this, and I assumed since i could not update over wifi, there was no further updates for my kindle, but I realized the latest firmware for my device was in fact 3.4.3. I found instructions on
this amazon page which also hosts the update files needed to manually update the firmware. I tried the process again at version 3.4.0 (still didnt work), and then eventually at the latest 3.4.3, when I stopped receiving the error, and KUAL finally opened.
This may not be the solution for every kindle, but with something as simple as updating the firmware jumping over my head for a week, I figured I would share my experience to anyone else who may be running into this issue. Check what firmware your kindle is running, update it to the latest, and try again. These devices deserve a second life!