Well, be honest about it. Is it diagnosed by a doctor or professional of some sort? Or is it more like, I occasionally read a number incorrectly?
If it's going to put yourself and others at risk, and/or it's documented by a professional, you definitely need to bring this up to your recruiter.
I looked at a Wikipedia page for Dyscalculia briefly, and the symptoms sound quite severe. Debilitatingly severe.
EDIT: After reading IRfly's post, he makes a good point about this and again I feel some of it comes down to your medical. You're going to fill out a lot of stuff for MEPS. They want to know your medical history, so it depends on how this has been diagnosed and handled thus far, and whether it's going to interfere.