Have you considered doing the natural thing and saying "OW!" when she does this? Not yelling AT HER, but reacting honestly to the pain and surprise? That's how kids learn to understand cause & effect - she may be too young to truly understand pain or to empathize, and your calm explanations aren't connecting with her at this age. Obviously it's not working to try to get her to have feelings for a live baby she can't see - I wouldn't emphasize that now, just focus more on yourself. You can hold her away from your for a moment and rub your sore belly, and she will get the idea that jumping on you DELAYS the hugs and attention. I also agree with the time out thing - separate her from you whenever she does this. If she gets attention, even negative attention (such as you explaining why it's a bad idea), it can sometimes have the opposite effect of encouraging her to do more.
I agree that you should protect yourself better, barricade yourself into a chair with a pillow on your belly, rather than be on a couch where she has better access to you. I agree with the other posts that truly hurting the baby is unlikely, short of her using a sharp object or pushing you down a flight of stairs.
Good luck!