I'm guessing you're asking the poster, but I got the juice by pureeing the watermelon in a blender and then straining through a cloth. You can squeeze out a lot of juice that way, just avoid getting too much pulp, its a pain in the ass because it'll just be more mess in your primary and it doesn't contain as much flavor.
My beer is about a week old in bottles (used DME for priming sugar, if it means anything) and it tastes sweet in the middle and finishes with an almost cucumber taste. Very smooth. I think its going to turn out really nice, though I don't really get any watermelon right now so maybe I did something wrong?