Hey don't worry. It does take *some* understanding. But you'll get there. Dukan is the only diet that gets easier, the longer you are on it.
I was "Dukan-ning" for a few months before I suddenly realised I was eating the wrong sort of yoghurt! I was choosing the 99.9% fat-free yoghurt, but failing to notice that it contained added sugar! Others may notice their own kinds of "fine-tuning" that they had to do.
Now, when shopping, I automatically check the nutrition panels of products and check the fat and carbs (per 100 grams, so it's a percentage figure). If it's more than 5% fat I don't buy it. "Sugars" doesn't always mean ADDED sugar because as has been pointed out, the natural sugars occurring in dairy products are permissable.
I found a whey protein powder that had almost zero fat and negligible carbs, and thought of buying it to use for a quick meal (with e.g. skim milk, an egg, splenda, a bit of fat-free cocoa powder etc) but decided not to, as it was pretty expensive and I figured real food was a better option!
The main things to remember are, ultra low fat and absolute minimum carbs. You'll get there! I have been on this WOE since October and am now about 37 pounds down!