Do you have a support contract? As far as I know it is available from their download area for Contivities, but like all hardware, you have to have a valid support contract or it will give you an "X" next to the link to DL it.
If you do not have a contract support, you will have to contact your Nortel vendor to purchase one.
If you do have a support contract, but still get the X, contact your local Nortel NNCDE or account manager and they can usually hook you up until you get the website/contract issues resolved.