Not right off the bat, that's for sure. Just like with so many other sorts of information. Nothing worse than being barraged with TMI upon meeting someone. So when? Early, but not too early. Get to know each other well enough to be able to see if the relationship has actual potential.
Personally, I threw it on the table when I had another "confession" to make. As long as I was at it, you know?
He was fine with it. Turned out he has mental "stuff" as well. Like girlwithbrownhair said, they've got secrets too! So true!

As for how... personally, it was something along the lines of "I feel compelled to let you know..." and that I was totally cool with talking about it or answering any questions he might have.