This guide will help you jailbreak iOS 4.2.1 on your iPad using Greenpois0n for Mac. You will then be able to install Cydia and use third party applications on your iPad apart from the App Store games and applications approved by Apple.
For a Windows Version of this tutorial read How to Jailbreak iPad using Greenpois0n [iOS 4.2.1] [Windows]
Please note that Greenpois0n does not update your firmware and you should first upgrade your firmware using iTunes first. In order to upgrade firmware download the .IPSW firmware file from here and then click Alt+Restore in iTunes(Mac) to update iPad.
If you don’t know what your current firmware version is or how to find iPad firmware version read How to Find iPhone Firmware and Baseband Version.
Steps to Jailbreak iOS 4.2.1 on iPad:
Point your browser to www.greenpois0n.com and click on the Mac link to download Greenpois0n tool for Mac.
After you save the archive and extract the files run Greenpois0n.
Once you have upgraded your iPad to iOS 4.2.1 as instructed above connect it to your computer using the USB cable and quit iTunes in case it is open. Power off the iPad by holding the Power button and then sliding the on screen slide to power off button.
The rest of the steps are self explanatory. You will have to press some buttons in the said sequence. Don’t worry about doing it wrong as the software will allow you to retry if you fail to put iPad in DFU mode the first time. Click on Jailbreak.
Upon successfully entering DFU mode the jailbreak process starts.
Click on Complete when Jailbreak is over.
At this point you will see some text scrolling on your iPad’s screen. Let it finish and iPad will restart automatically.
After your iPad restarts you should be able to see a new Loader icon on your homescreen. Tap on Loader to launch it. We will use it to install Cydia on iPad.
Tap on Cydia and then tap on Install Cydia. Post installation press options and then tap on Remove Loader.app to remove this app which is not required any more.
You now have a Jailbroken iPad running iOS 4.2.1 with Cydia installed. Easy right?