I find myself to be very approachable (mainly cuz I'm a chatty kathy) so I'll do my best to give you some good advice:
The first thing you have to do to become more approachable is you have to FORCE yourself to be outgoing, optimistic, and open to meeting any and all sorts of people. So when you are out at the clubs, get a drink, talk to the person next to you (or if you see someone you fancy, approach them). Being approachable is all about confidence and being able to approach other people. I'm not sure why people think that everyone should approach them-- sometimes the other people are just as scared and so if you approach them it shows an aura of confidence.
You need to smile, crack jokes, and you need to be able to hold your own even if you feel out of your element. Meeting new people isn't that hard if you have a good sense of humor or wit about you. Usually people start talking over a joke, or something funny that happened. If you see something silly or outrageous at the bar, comment about it to someone- i guarantee they will say something back, and a conversation may ensue.
Get out and dance, mingle-- none of that wallflower BS. If you stay on the sides watching everyone that's having fun, you'll always be doing that- watching. But if you get out and participate and dance and have a good time- people will approach you.
It's about being seen. If people see you and see you having a good time, they will approach you, and you'll also gain confidence to approach them. If you stand on the walls, no one will notice you.
You get what you give. Especially in gay bars. Best of luck!!!