Yes, you can have ranch dressing, and don't worry about carbs. Instead, look at the fat and sugar. You want something with 3g of sugar or less, and you want the fats to come from canola oil, olive oil, or other healthy oils. If it has the word "hydrogenated", put it back. I've found Hidden Valley's ranch dressing all seem to conform to that rule, but experiment...there are lots of dressings out that that are allowed.
And definitely eat! Are you getting enough veggies? You need to eat lots on this diet...try peppers dipped in hummus, celery with laughing cow cheese, steamed broccoli, green peppers with slivered almonds, jicama dipped in salsa, etc. There's a lot out there!
Make sure you are eating every 3 hours to keep your blood sugar stable, and that you eat when you are hungry.