You can only unlock an iPad by restoring it using the computer with which you last synched it. This allows you to reset your passcode and re-sync the data from the device. You can as well restore it in a different computer although your data will not be present. To restore, you will need to have the latest version of iTunes. Insert a non accepted SIM card and Connect the iPad to iTunes (IE not from the original network provider) and wait until iTunes detects your device. After about 20 seconds, disconnect your iPad and you will receive a message showing your success in unlocking your device.