All of my bettas have refused to eat when I first brought them home. Its the stress of being in a totally different environment, plus being transported, that makes them loose thier appetite. I found that a great way to entice a betta fish to eat is to get some frozen blood worms. After I unthaw them I get the end of a blood worm stuck to the tip of a plastic spoon. Since frozen blood worms are wet, they will stick to the spoon. Then I drag the other end of the blood worm through the water (just be sure not to submerge the spoon, or the blood worm will fall off) The movement of the blood worm simulates live food and is a lot more attractive to a betta. Its worked like a charm every time my bettas don't want to eat.
I hope your betta starts eating soon!