These are instructions on how to jailbreak your iPad 1 on the 4.3.1 firmware using RedSn0w for Mac. The Windows version of this tutorial is here.
Step One
- Download RedSn0w from here
- Download: 4.3.1 (iPad 1): iPad1,1_4.3.1_8G4_Restore.ipsw
Extract the RedSn0w zip file by double clicking it and Launch the redsn0w
Step Two
Once RedSn0w opens click the Browse button 
Step Three
Select the 4.3.1 firmware ipsw then click Open. 
Step Four
Once the firmware has been verified click the Next button to continue. 
Step Five
RedSn0w will now prepare the jailbreak data 
Step Six
From this window you can select the jailbreak options you would like. 
Make sure Cydia is selected and click the Next button to continue
Step Seven
Please plug your iPad into the computer and make sure its OFF then click the Next button 
Step Eight
RedSn0w will now guide you through the steps to get into DFU mode. 
Hold down both the Home button and the Power button for 10 seconds. 
Release the Power button and continue holding the Home button until RedSn0w detects the device. 
Step Nine
Your iPad will now reboot 
Step Ten
RedSn0w will then begin uploading the new RAM Disk and Kernel. 
Step Eleven
Once this is complete you will be notified that RedSn0w is done. Click the Finish button. When your iPad finishes rebooting (5 minutes or so) it will be jailbroken with Cydia on the SpringBoard. 
*As usual a big thanks to the iPhone Dev-Team and I0n1c for their hard work and contribution to the iPhone community.
