Berat: The use of "at" vs. "in" is, indeed, confusing. One excellent teacher at this website told us this "rule": AT is for place; IN is for space. In other words, if you are referring to a location, most native speakers would probably prefer ":at." Tom: Where shall we meet tomorrow? Mona: Let's meet AT the parking lot on Maple Street. If -- for some reason -- the inside of the parking lot is important, then you could use IN: There were so many cars IN the parking lot that I forgot where I had parked my car, so I had to walk around IN the parking lot for 30 minutes before I found my car.

None really, but it's usually two words, "car park", not "carpark".

Basically, it's the difference between "in" and "at", isn't it? :) "In" is a point surrounded by limits. The limits can be walls, borders, or anything else that defines a "space" "At" is a point in a journey through time or space. If you go to a place, then you are at that place. If I am "at the carpark", that means I have arrived there, and maybe I will go somewhere after that.

In the USA it is usually called a parking lot, a garage, or a parking structure depending on whether their are multiple levels and/or if it is underground. Cultural note.