Approachability is a matter of perception. Even if you think you're approachable, people around you might not perceive you as such. There are lots of factors involved, with the body language being the most important one.
Basically, if you look at people with "what the fu*k do you want from me" look in your eyes, accompanied by an appropriate "gtfo" facial expression and posture, you can think of yourself that you're approachable as much as you want, but people will perceive you as intimidating and unapproachable, and act accordingly.
On the Internet, where the body language is taken out of the equation, people will judge you based on your e-manners, on your posting style, how you express yourself and your general demeanor. Every single detail counts, no matter how small and insignificant you might think it is.
What you do with your profile settings also counts. Your avatar, signature, user title, etc. If you want to look approachable, avoid using images of someone or something that looks serious or angry as an avatar, for example. Signature should also contain something that doesn't scream "I'm better than you" (not literally, of course), or can be perceived as such. This especially goes for people in power, like forum moderators and administrators, since looking intimidating and unapproachable won't really help attract members to their communities, nor keep them from leaving.
Of course, different people perceive same things in a different manner, so you'll never be able to please everyone, but if you want to avoid having the majority perceive you as intimidating and unapproachable, don't act like that.