I suggest a simple straightforward approach. Walk up to him, smile, and say "Hi." The smile is important. That should be enough. If the guy doesn't seem thrilled, you could add "You look like you could use some company," or a similar supportive statement. A compliment might be a good opener. For example, a woman once told me I had great legs. That really impressed me. I once had a woman stare at me with an inviting look. I turned to look behind me, wondering if she was actually looking at someone else. I hesitated, concerned that I might be somehow misinterpreting her stare. I finally approached her and the outcome was most pleasant. Traditionally the burden falls on the man to initiate contact. It can make a guy feel like a door-to-door salesman, a job that many men are not cut out for. I believe men generally appreciate an approach from an assertive woman. Be prepared for rejection. It's bruising, but we all go through it. (Incidentally, I think clubs are terrible and recommend other venues for meeting people.) Please bear in mind that the above statements are not meant to be universal, all-encompassing truths, only feedback from one observer.

Catch his eye and hold his gaze for a few seconds- its not too hard, just look at him from acroos the room and wait 'til he turns around, look for a few seconds, but turn away or drop your eyes in a coy way before it seems too much like staring. Also check out that he doesn't have anyone with him who might be his other half. A little later, do it again, only look for longer this time. Then go for a dance, let him see you enjoying yourself, then do it again (preferably as you move off the dance floor), but this time smile, wink, or even blow a little kiss. If he smiles back you're onto a winner, if he looks away, chances are he's attached (or otherwise not interested)- make an excuse to walk past him (on your way to the bar, the bathroom, or to talk to someone else) and as you go past brush lightly past him and whisper something flirtatious in his ear ( "I'll be back for you later honey" works for me, but you might want to use something a bit less cheeky!) but keep on walking. He may well come and find you after that, but if not try buying him a drink (or asking someone else to) and just say "I got a little tired of waiting so I thought I'd break with tradition. Hi I'm...". Its only an example obviously- the main parts are making eye-contact, assessing his possible interest and getting the flirtation going. There are infinite variations. If you're a really shy type you might find it easier to make an excuse to talk to him, such as bumming a cigarette, or asking the time, but I tend to find in a club environment most guys are pretty open to women approaching them for no particular reason.

you don't have to exactly approach simply gaze at him and smile or something else more convincing i guess.