If you're a huge skater, you got this one down perfectly. If not, see below:
Try and seem interested in what he loves. Smile and say 'awesome' or 'cool' when he does something really impressive. He is not doing these to impress you though, so don't overdo it. Try it casual--Be friendly and flirty. But not too flirty...most skaters don't like clingy and super flirty girls.
Ask questions, it will show him that you're interested and really care about his passions. He should always feel like you support what he does. Maybe he will learn to do the same for you.
Be brave, guys like confidence! Ask him to show you how to stand on a skate board or how to go. Remind him you're a beginner and don't want to do any advanced things. Skateboarding is a really creative hobby, and he will be proud when you get good! (Make him hold you while you're trying to stand, guys like that.)
Make sure the guy you like is really worth it. You should like him for his personality, not just the way he skates. Get to know the guy behind the board, outside of the skate park. Ask him about himself, and not just his skating. Otherwise he may think you're not as interested.
If he hasn't seemed to notice you, you should maybe try complimenting him on the way he skateboards or watching him quite often and smiling whenever he does a cool trick. Laugh and be approachable, he'll come to you.
If you know how to skateboard, do some tricks in front of him. This might get him interested in you. If you're really good, impress him, but be modest about it.
Try getting to know some of his friends, that way they can introduce you to him. Show your interest in skateboarding, and one of his friends is bound to notice.
Don't lie about your skater skills, EVER. If you say you can, and you really can't, he'll think you're a poser. Be yourself, and not a sk8r wannabe.
If he still hasn't noticed you, he probably just isn't interested. You can't catch the attention of every guy.
Ok FINE I didn't write this (I would never say "sk8r wannabe"). I also did not google this (but I would), it was stolen off an old pal's Facebook page. That being said, yeah I posted this so feel free to tear into me in the comments (we all know the comments are the best part of this blog anyway)
REMEMBER: "If you do a kickflip, the guy will fall in love with you automatically. It's a known fact."