To look approachable you probably need to FEEL approachable. Which means, getting over that urge to put a gag on the person's mouth anytime he/she comes near you. Find a way to get interested in all that he/she says.
The body language reflexes that accompany approachability include a relaxed stance (no crossed arms, no hunched shoulders), an eager or calm face (when you are lost in though, or look stressed or angry, no one will want to come by), a quick acknowledgment when they pass by your desk, and so on. Make eye contact; smile briefly.
I do have to admit, however, that Tahiti has a point. It can be difficult to fake it, and it can be costly to you as an individual. Perhaps fake it long enough to leave of your own choice? Then find an employer that doesn't place so much value on this one trait.
_________________
Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).