I believe FloExpress is only intended for studying fluid flow inside objects, not around them, which is why it might be asking for the lids.
Have a look at the FloExpress tutorial within SolidWorks. That should clear up your 'Lids' query.
Basically, your car body needs to sit inside a 'wind tunnel', so if you need create a tunnel around your model, with an open front and back.
You then need to block the front and back holes up with 'Lids' (basically, convert entities of the inner profile of your hole, then extrude and uncheck merge results, then assign one lid as an inlet, and one as an outlet.
Please see attached image.
Please someone correct me if I am wrong, the only time I have used FloExpress is when I completed the built in tutorial when I first got SW last year.
FloExpress.jpg 143.1 KB