You will need to enable USB Tethering for your Motorola XT875 to be used as a modem. (Account provisioning may be required, please contact your service provider for this feature)
To enable this feature:
Touch Tethering & Mobile Hotspot
Check the box for USB Tethering (this option is not selectable while Wi-Fi is on, or unless connected to a USB cable)
You can tether your Motorola XT875 to your computer with a USB cable to share your Motorola XT875’s internet connection with your computer.
Tethering works with Windows Vista, Windows 7, Windows 8 and Linux.
For more information, including using USB tethering with other Operating Systems, see www.android.com/tether#usb