I don't know; I've always been an advocate of getting to know someone as a friend first, but if it's a dating situation right from the start, it's important to let the other person know early on that you're poly. I suppose you could just say, "You know, I should let you know that I'm polyamorous." Then let them take the lead on where the conversation goes, such as if they don't know what polyamory is, in which case you can describe it to them.
I'm not as familiar with POF, but OKCupid has a pretty good reputation for being poly-friendly. Plus on OKC you can put right on your profile that you're poly, and explain what that is. That way anyone who looks at your profile will already know.
Hope that helps,
Love means never having to say, "Put down that meat cleaver!"