Both are correct. If you use "in", then you are inside the park. If you use "at", then you are either inside the park or very close to it, perhaps standing at the entrance on the road outside. Here's an example dialogue for you: A: Where are you and what are you doing? B: I'm at the hospital smoking. A: You're smoking in the hospital? B: No, I'm not in the hospital. I'm just outside. You can't smoke inside.

Both are correct. The first sentence we would use more as a general statement. If someone were to ask you where you are, generally you would say "I'm at the park" or "I'm at school", etc. "I'm in the park" you would use if you wanted to say exactly where you are. Normally we just use "at", especially if we are talking about an open space.