Welcome to SparkPeople! There are a few different ways to go about this, and as you get adjusted to the system you should find out what works best for you. One thing you can do is track your exercise in advance. If you know that you are going to an exercise class (or swim), you can enter that in the morning and your nutrition calorie range will automatically adjust. Even if you don't know the exact number of calories you'll burn later in the day, if you know (for example), that you will swim for 1 hour, you can enter that into your tracker. You can adjust later if need be, but most likely the estimate will be similar to what you actually burn that day.
If you find that your exercise varies too much from class-to-class to estimate reliably, you can also switch to our classic mode of tracking calories. That method uses your calories burned GOAL (not calories tracked) to estimate your nutrition range. The downside to the older method is that your calorie range will not adjust according to what you track and the number for your nutrition will be the same each day regardless of activity. If you decide to go that route, we recommend that you make sure your calories burned goal is as close as possible to your actual activity level each week.