The only way is to be honest about it, and hope your interviewer is understanding. However, you have 30 minutes to make an impression, and you don't want a majority of that time to be your trying to rationalize a screw-up.
Just go in and be yourself. If you don't know something, say you don't know. Nervousness should hopefully diminish if you're not misrepresenting who you are, and you won't have any problems with lack of understanding if you don't try to act like you know something you don't.
Hell, at a large number of schools, statistically, the hardest part of the process is getting to the interview, not getting accepted afterward. Just be yourself.