Are you doing it on a treadmill? If so, use the calories it records.

If not, you'll have to add the minutes run (at the approx pace) and the minutes walked (at the approx pace) manually. There isn't a C25K exercise on MFP.

You'll have to adjust this calculation to your walking and running speeds:

For week 1 day 1, there are 8 x 1 minutes of running and 8 x 1.5 minutes of walking plus 10 minutes of warm up & cool down walking. Totals are 8 minutes of running and 22 minutes of walking.
Using MFP's database, there is no running below 8.4Kilometers per hour and you aren't running that fast yet (or shouldn't be). This is not a discouragement or judgement. I used this method myself when I first started running. I entered the speed I used on the treadmill and then entered the calories from my treadmill display. But my speed was the actual, average speed.....walking! It's a thrill when you get to log your first "run".

For now, you'll have to either choose a walking pace for the total 30 minutes or, if you want to divide the sessions, find a pace for "walking" for the 8 minutes of running and a slower pace of "walking" for the 22 minutes. Something like, "Walking, 15 mins per km, leisurely pace" for the 8 minutes of running (MFP will give a calorie burn when you enter the time) and "Walking, 18.5 mins per km, slow pace" for the 22 minutes of walking.
Better to log a bit under your actual burn than a bit over if you're eating back the calories. It's all an estimate.

Are you running outdoors? A Garmin watch (or any other GPS watch) will log your calories burned, too.

Just more information... from my experience and especially important if you are eating calories back. When I started I was using the calories burned from my treadmill. Then I got a heart rate monitor and sports watch and the calories burned were significantly lowed with the heart rate monitor estimate.