Your SIM card has to be specific to the carrier which your xoom is assigned to; optionally you can unlock your xoom (use a web site to buy a code or some kiosks in some malls offer this service) and then use any SIM card with a Data plan.
I bought the xoom and it has the slot for the sim card so I put mine in and it did not work. Why? Can anyone help me?
A SIM card will only work in the MZ601 model. Check in your settings / About tablet, and verify the model number. Anything apart from MZ601 will not work with a sim.