If you bought your Xoom from Verizon, it will have no SIM card slot. Verizon's CDMA network doesn't use SIM cards. Also, if you bought the Verizom model, it will only work on Verizon, and not another carrier. There is also a WiFi-Only Xoom, and it does not have a SIM slot.

I have a wifi model and it *does* have a sim slot, in the same place as the microSD slot. Neither of them work on mine....--yet--

Mine is a referbished Verizon model. It has wifi and an empty sim slot by the SD slot. I only use'wifi for now but when I was talking to Verizon they said I can get 4g on it. Also, on my wifi scan screen it always shows me the Verizon carrier and strength. I also think that my map/GPS can use the Verizon network to download maps even though I don't have service because no matter where I am I can literally watch the map fill in as it downloads.

so are u meaning to say, the xooms that has gone tru a 4G LTE upgrade, are capable of running a 3G sim card overseas...?

The SIM card slot is right beside the SD card slot. They are accessed behind the same "door" on the device. The SD card and SIM card fit almost back to back.

The SIM card rests in the door/carrier when installing or removing.

See the picture here: iFixit Xoom picture