About OKCupid: I feel your OKC pain, my husband has had a difficult time, but I have encouraged him to message many women, understanding that only some will be interested enough to reply. Being upfront about being poly on there is the best thing to do! Kudos! You would hate to lead someone on and waste yours and her time.
Also it has been my experience that many of the OKC girls are pretty defensive about the whole thing. "You can message me if: You don't mention my pictures, you leave the perfect message, if you are completely perfect in every way" Ok, not that bad, but sometimes they can be. I am on there too, but I have had a wonderful time meeting guys, much easier. Just keep in mind that not everyone will be alright with a poly guy. In fact you could do a keyword search for "poly" and see what members pop up, then message them just for conversation (my husband did this and has a few girls who love chatting with him).
NYCindie is right, you are a OKC newbie, give it time! Also, make sure you are putting in an effort to send out many messages. It is like fishing, the more lines you have in the water the better chance you have at catching something (I think).