How to gain weight for the Army?

So I went to MEPS yesterday to do my ASVAB and to do my complete physical. Everything was good to go....until I had to weigh in. I am 6 lbs UNDERWEIGHT. I have to go back in 24 days to weigh in again, so I have until then to gain 10 lbs (just to be on the safe side). I was wondering if those Carnation chocolate/vanilla/strawberry drinks will help. Anyone know? Thanks for the advice.