How to retrieve forgotten passcode??

The only way is by remembering it - it's not stored anywhere. If you can't remember it then you will need to connect the iPad to the computer that you normally sync to and you should then be able to reset the iPad and re-sync your content to it (http://support.apple.com/kb/HT1212) - you may need to put the iPad into recovery mode : http://support.apple.com/kb/ht1808

If you do it via a different computer then :

If you restore on a different computer that was never synced with the device, you will be able to unlock the device for use and remove the passcode, but your data will not be present.

If you cannot remember the passcode, you will need to restore your device using the computer with which you last synced it. This allows you to reset your passcode and resync the data from the device (or restore from a backup).

If you restore on a different computer that was never synced with the device, you will be able to unlock the device for use and remove the passcode, but your data will not be present.

You may have to force iPad into Recovery Mode

http://support.apple.com/kb/ht1808