assuming you DO arrive by 7:15am I think you'll be fine?
use hopstop.com
It says from penn station to nyu dental (345 E 24th street) its about 30 mins (including walking time).
A cab takes ~15 minutes (but more expensive).
I'd take the cab option in case you're not too savvy with directions. (Its really easy to get to nyu dental though from the 1st ave stop though).
Good luck! And find comfort in the fact that I got there early (7:40) and was immediately called to interview (it lasted about an hr for me, less for others). So if you arrive a bit late, your interviewer might not notice. 🙂