In the address field just put "HOLD AT MEX" and not an actual address. When it arrives there IB's handling agent will contact you to either arrange for you to come and collect it by being escorted into the bag hall and you walking it out through customs yourself (not very common but maybe more so in South America), or they will provide you with a customs declaration to return to them before it goes to customs to be inspected and finally you collect it from the handling agent's office.