Could you please look at the next picture in the tutorial? The picture you shared (10. Streamlines 2) actually refers to adding the front wing to the viewer. But to be able to load streamlines you need to click on the solution field that has the fluid internal mesh loaded. So to do this you need to click on the Run-Standard where you have the slice and the calculator, and THEN you can load the streamlines.
Please let me know if this works