If you have setup a MyiRex account on that iLiad, then you can go to the
https://myirex.irexnet.com/user.php website, login, and click on "Reset PIN code". Then press the connect button on the iLiad and it'll sync with iDS and clear the PIN for you.
If you don't have a MyiRex account setup yet, then you will probably need to contact iRex and ask them what to do.