Double check that you’ve followed the instructions on How do I prepare my 3D file to print?
The first option is to connect your computer to the printer through USB and click “Print” on your prepared file in Makerbot Desktop.
The second option is to take a USB drive with your file on it and plug it into the Makerbot. On the Makerbot screen, navigate to “Print,” select it, then select “USB Storage.” Find and select your file and start the print.
The Makerbot will automatically heat the extruder and begin printing when ready. Be sure not to leave a print completely unattended, as filament jams and other problems are common.