You must first enable Ground Freight Pricing in the UPS Settings pop-up (setting > Carriers & Fulfillment, then edit your UPS account settings).
The Ground Freight option is in the "Other Shipping Options" section of the shipping sidebar.
Just check the "This shipment requires a Freight Classification code" and then enter the freight classification code that applies to the shipment in the field that appears.
UPS Ground with Freight Pricing is only available for multi-package UPS shipments that use the shipping service UPS® Ground.
When you have crated the order. In the "Configure Shipment" section where you can change the dimensions, and the weight of the items you are creating the label for. You will see a drop down menu labeled "Service" there you should be able to see a drop down menu with all available carrier and their different services like Ground, 2 day, etc
Just select the one you need and then finish creating the label. You can also have certain items be defaulted to UPS Ground or any other service and then if an adjustment is needed you can change it from there.
Hope that helps!