Everyone was pretty excited when the untethered jailbreak for iOS 4.2.1 came out in the form of Greenpois0n RC5. However, that excitement soon fell into a puddle of disappointment when many encountered problems while executing the jailbreak process, from some weird verbose errors to sudden crashes of the Greenpois0n program.
The following method to make it works is applicable to the Mac platform and it has been confirmed to work (at least on my side here who have encountered those earlier problems). And I have tried it on iDevices which were both freshly restored or just a direct jailbreak on a system which is already on 4.2.1 and even over a RedSn0w jailbroken one.
1) Open terminal
2) Key in "sudo " (note the space after the command)
3) Right-click on the Greenpois0n app, select "Show Package Contents"
4) Go deep into the MacOS folder and drag the "greenpois0n" app into the terminal
5) Press "Enter", enter your password and proceed with the jailbreaking process
It should work now. The same verbose errors might appear again, but as you will see in the terminal, the jailbreaking process is actually going on :)