A general rule is to figure out how many calories you need, then add 500 for milk making. That 500 number is just a general guideline- newborns might take less than 500 calories of milk per day, an active 5mo not yet on solids might take more, etc. Are you supplementing with formula, or is breastmilk the only milk your baby gets? If you're supplmenting, you should figure on breastmilk taking fewer calories from your body. I personally never counted calories while nursing; but I made an effort to eat when hungry and stop eating when full.
As for holding onto weight while nursing, it really is an individual thing. I know my aunt held onto an extra 5 lbs while nursing, which she quickly lost within a couple of weeks of weaning- but then again, she was literally only 5-10 lbs overweight while nursing, so once she weaned she was back in her pre-pregnancy clothes.
I personally found that I was able to start losing weight fairly easily once my babies were about 8-9mo, even while still nursing. Some weight dropped in the first 2 months, and then I basically maintained until the baby got bigger and I was able to get more active, watch what I ate, etc. By the time my son was a year old, I was back to the weight I was when he was conceived. Of course, THAT weight was still 40 lbs heavier than before I conceived my oldest.
I think having my daughters too close together (16.5m, I found out I was pg again when my oldest was 9mo) messed me up, as I didn't have time to lose any weight between pregnancies. 12 years later I'm working on those pounds.
I suggest you have some patience. You'll probably start losing about a pound a week pretty soon, just doing what you've been doing. If not, take a closer look at your diet: are you eating too many carbs? Too many calories? Too few? Plus, don't forget to check your inches/how clothes fit. You might be getting trimmer even if the scale isn't moving.