The image below is of the background on the middle 3D computer station. You will see it has an image of each machine this computer is able to print to and corresponding instructions underneath. As this is the background image to the screen these instructions will always be available to you.
For Ultimaker 3D printer you will need to double-click the "Cura" icon to start the Cura software.
Once the Cura window opens you can load your model in two different ways. As seen below, if you have your .stl on the desktop or in a File Manager window you can click and drag the file into the 3D plane. Otherwise, you can go to File -> Load Model File and select your .stl.
After your model has loaded into the 3D plane, click on it to select it. Once it's selected it will acquire a blue outline, and the icons for Rotation, Scaling and Mirroring will no longer be grayed out.
When you are ready to print, click the 'Print via USB' button in the lower right. If this option is unavailable from the drop-down menu, it means that the program is not properly connected to the Ultimaker 2+ Extended. Ask the technician on duty for help.
Cura will automatically switch to the 'Monitor' tab, where you can see the progress of your print. Don't worry if it doesn't start right away - it takes a couple minutes for the nozzle and build plate to heat up.
Please be mindful of the time your model needs to print. An estimate will be displayed in the bottom right corner of the 3D plane whenever you make a change to the model. Time to complete should be before the library closes.